I think
this could work, you'd just have to re-position the clips and straighten the curls out a bit. If you order it now, you should get with time to spare for Otakon, I ordered a wig here recently and it took maybe 2 weeks to arrive using (free!) standard shipping.
If you want to stick with US sellers, I'd recommend
Arda Wigs, buy a short wig + 2 ponytail clips. I really like the Rose Pink color, though Bubblegum Pink might work too! Epic Cosplay is also worth checking out, but
this would require some custom dyeing. They have other colors that would work better for Momoka, but you'd have to stub the wig which is really annoying. ^^; Anyway, I hope this helps~