What to do in case of zombies?

Solid plan man, except for a lack of an exit strategy. Instead of Sam's Club, it's Costco for me. (except a lack of guns/ammo.) It's probably one of the more realistic plans out there. Since my Costco don't got guns, I'd probably procure from somewhere and then head over to the Costco. I should make a note that I'm not expecting slow shuffling zombies. I'm expecting zombies that can run and do whatever it takes to get you like Left 4 Dead. Here's a list stuff I would bring.

Things I would bring along: (in order of importance)
1. First aid - a fucking must, unless you get bitten
2. Flashlights and spare batteries (not attached to gun/s) - when we talk zombie apocalypse, most likely many generators, power plants, service towers will stop functioning, and there's a big chance they will. This will help illuminate your path. Flares/glowsticks can be used as an alternative, but best used for temporary lighting and signaling other survivors.
3. Extra rations - to survive the trek to Costco/gun store in case shit hits the fan, or any trek. MREs are obviously recommended, but any other non-perishable food is a must unless you find a fridge of frozen food, e.g. snack bars, bags of chips, canned food, etc. Canned food can be open with a swiss army knife.
4. Radio, walkie talkie, cellphone - the cell in the first few days/weeks until the service towers stop function. Obvious usage for contacting survivors, armed forces, or for SOS over long distances.
5. Belt holder/holsters and backpack - to hold your shit and weapons of course.
6. Matches/lighters/lighter fluid - another source of light, but also can be used to cook food, light molotovs, and signaling other survivors.
7. Body armor and helmet - light, but durable is key. You want to be mobile if zombies start gunning for you. Arm guards, shin/knee guards, neck protector, boots, and some sort of chest/back armor. Kevlar can be used as I would assume it can withstand a bite.
8. Clothes - as funny as this may sound, you might want an extra pair, two, or three sets of clothes for wear and tear, dirtiness, and if you get blood/vomit/something-strong-smelling on you, you should ditch those clothes because the smell will attract zombies.
9. Personal hygiene kit - optional, but very common sense. Only if you have room in your luggage/transport.

Weapons/armaments: (with flashlights attached)
1. Melee weapons: when the shit really hits the fan, I'd go with either a katana (unlikely but nice), lead pipe, crowbar, aluminum bat, knives, or machetes. At the very least, have a swiss army knife to do various functions. Axes or chainsaws are cumbersome and impractical. Always aim for the head/spine.
2. Handguns x2 w/ silencers: in case I lose my primary weapon, anything from glocks to berettas to Heckler Kochs, though penetration is more important than magazine capacity I believe. Revolvers and magnums should not be on your list of handguns to procure simply due to small magazine capacity even if they offer max penetration in the handgun family. Colt M1911A are fine as well. Always aim for the head when possible - a spine shot is too risky.
3. Primary weapons: there are 3 categories of weapons to choose (Left 4 Dead experience)
a) Shotgun (semi-auto): good for close range, close quarter combat inside buildings, rooms, alleyways, narrow streets, and forests/woods. Semi-auto (benelli M1014) is preferable to the pump-action (Remington/Mossberg) due to less action involved every time pumping a shell into the chamber. Also, the spread pattern and force that a shell spews out is very lethal, and at 1-10 yards it is guaranteed a 1-shot KO. Little aiming is required, but chest/headshots are your best bet. Beware of recoil.
b) Assault Rifle (AR): good for medium range combat (also for close/long) such as streets, open country, towns, and even inside buildings. Recommended model is M4 Carbine with SOPMOD packs if possible - better than M16 because usage can extend to close-quarters with less barrel length for turning angles. Also, M4 is the popular variant of the M16, and most used AR model. Good for spraying and blanket/cover fire, but that is not recommended unless pressured. Use burst fire to conserve ammo. A subcategory is the SMG, but in a zombie fight, aim and precision will do you much better than wildly spraying bullets into the zombie horde while wasting ammo hoping to get headshots, which is what the SMG was designed to do. Always aim for the head when possible - a spine shot is too risky.
c) Sniper/hunting rifle (semi-auto): you want this for range, and long range only. Picking off zombies at a distance is much safer than tangoing with them at close range. Also, bullet velocity of most sniper/hunting rifles should give you more than enough penetration to go through multiple zombies at once since their flesh is decomposing/decomposed. You also don't want to be bolt-feeding every round into the chamber when the zombies start running after you, so a semi-auto sniper is your best bet. Prefered range for this weapon at over 800 to 1200 meters. The M21 sniper rifle is recommended. (again, Left 4 Dead speaking here) Also, if you can't find a semi-auto sniper rifle, your next best bet is to find a sniper rifle that can hold a magazine of bullets instead of a 1-bullet holding capacity rifle. Always aim for the head when possible - a spine shot is too risky.
4. Grenades: flashbangs are useless since zombies don't hunt by sight. Recommended molotovs for close quarters and pipe bombs for out in the open with a alarm sound attached to it to attract the zombie horde. (Left 4 Dead again - these guys who made this game knew their shit) Frag grenades are viable if on hand, but unless the zombies are sitting/standing still, precision aiming of the frag grenade will not be easy once zombies start running after you.
5. Special weaponry: not recommended, but if you can, keep it in stock. Flamethrowers, machineguns, and grenade launchers, with common sense application.

As for mobile transport:
1. Motorcycles: Fast, agile, versatile in any place for one or two riders except in mountainous regions.
2. Cars: probably the most used variant of transport, but armor-plating it becomes important to keep zombies from busting it. This option should be avoided due to the sound a car makes. A motorcycle or SUV would serve better.
3. SUVS/jeeps: more horsepower than a car with the advantage of being able to traverse in any terrain except underwater. Reinforce with armor.
4. Armored transport: Buses, trailers, trucks, (all armor reinforced) and military transport such as APCs and even tanks for containing and moving many survivors over long distances.
5. Air transport: helicopters, biplanes, airplanes, and armed jets - don't ever have to touch the ground except to refuel and resupply.
6. Water transport: trawlers, fishing boats, anything with a motor - keeps you away from the zombies even if they can swim for a little. Better off the ground when possible. Good for transporting many survivors as well, and catching fish shouldn't be a problem.
7. Bike/your feet: most dangerous option, but if the shit hits the fan, you'll be doing this. Always be aware of your surroundings when you're on foot, because once you get swarmed with no place to go and not enough or no ammo, you're fucked.

Last but not least, an exit strategy. Ultimately, food will run out in Costco, so it becomes imperative for some sort of farming community to be erected for sustenance with obvious defenses like walls and gun emplacements, either in city/town, near city, or in the open country. This will only take place if its a large community of survivors with enough equipment, cars, supplies, and rations to make such a trek.

***** I pointed out things that I would recommend for weapons - understand that they are recommendations and I don't think the local gun shop will stock M4s and M1014s and M21s. I mean, that's top of line firearms. More reasonable recommendations would be the pistols that I recommended, melee weapons, and list of supplies to gather. While I don't think gun shops will stock military-grade assault rifles or semi-auto sniper rifles, they do provide the regular hunting rifles, shotguns, pistols, and assault rifles (AR-15s). I mentioned before the Remington and Mossberg shotguns because those are the two mose popular models out there that provide quality. After some research, I found out that Benelli shotguns are sold online and in gun shops, so that's your next best bet in semi-auto shotguns. The Benelli M1014 won't be available, but the Benelli M2 Field is. While M4 ARs aren't available, the AR-15s and PDWs (like the submachine gun MP7) are available for sale.