There used to be a site for exactly this called custommade.com. Aside from a list of items that others have already made for you to browse, you could also put up a description of the project and let craftsmen contact you with questions or bids for the project.

I'm not entirely sure what happened but they ended up refocusing on strictly custom jewelry instead of custom whatever you want that someone else can make. It's really unfortunate because it was an excellent service with very talented makers and I haven't been able to find anything similar to replace it.