Itch.io is a bit of hit and miss. I've noticed that people offering money for the job get inundated with responses, and people offer to do it for less to get the job. The majority have people contact them on Discord to continue a conversation - but sometimes the conversations from Discord filter back to here. A case was where someone had recruited a team, and offered to pay a graphic artist, but then killed the project. The graphic artist had already done the work...
Payment is between you and the person you're hiring. It's difficult on a platform like Itch.io, because you both have to trust each other with nothing guaranteeing that trust. If you go to a site like Fiverr or Upwork, you have some protection.
Where you might have some success is that when you browse the assets, there are developers that have posted free or paid assets but also offer their services for custom jobs. Perhaps search for the kind of asset you want, and look at the pages that come up in the search results.