I commissioned some images for a long story I posted here called "Temple of the Torture Goddess."
When I was looking for artists, I just made a list of artists who post here, and some artists I liked on deviantart (even a few that didn't focus on tickle-related stuff), and just contacted all of them privately. I told them what I wanted, asked if they did commissions, how much, delivery time, terms, etc. (Some people post that info on their deviantart page) and we went from there.
Almost everyone got back to me, and all the ones I ended up commissioning were professional, accommodating, and easy to work with. Even the ones that weren't interested for whatever reason were nice about it.
So… I guess the short answer is, find them, reach out, tell them what you're looking for, and go from there.