Having a non decorator design a cake is usually a bad idea, they have no idea how much time different details take, or how to balance a cake, etc. So I wouldn't suggest craigslist.
I really don't think you will find a decorator who will design a one of a kind cake and sell you the sketches, at least not for a price that would make it feasible for you.
I have times when creativity does not exist in my little world, lol. There was a thread on here not too long ago about where people find inspiration. I would try searching CC for threads like that, they are really helpful.
I keep pinterest boards for motivation, everything from fabrics to landscapes that I like.
I would suggest finding a few pictures you love, of anything except cakes, and just start sketching them into a cake, until you come up with something you like.
I do that all the time, and it really does help me. (Even if 99% of the sketches look like crud, lol)
Also, instead of taking an entire design, try just taking small components and piecing them together.
Some people are just amazingly creative and have a solid vision for what they want, for some of us, it's a real process.