I don't know about the Japanese eromanga artist scene, but you're probably most likely to find someone willing to work with you by reaching out to people directly on social media with proposals.
Many artists have their own stories that they work on, so you need to meet someone that is willing to illustrate such ideas that aren't their own. Maybe they're interested in the practice or just don't have many original ideas and are looking to expand their skill.
I say, just look at some independant 18+ artists, check their profiles if they're specifically taking commissions, and if they are then ask them if they would be willing to illustrate your story. Then you can work out details from there if they show interest. Ask them what they feel is fair compensation and don't argue with them. It takes a long time and a lot of hard work to create an illustration, much less an entire manga's worth of a story, so if you cannot afford what they ask for, just thank them for considering it and move on to someone else.
Good luck finding someone to assist you.