I was listening to a On Board Game podcast earlier today, and their guest was Mike Mihealsick, founder of Coalition Game Studios. His company plays prototypes and gives you tons of in-depth feedback. There are multiple different packages, and it all sounds reasonably affordable.
I think I'm going to use this service when I'm a little more confident in my design! I'll let you all know how that turns out whenever that happens :)
In the meantime, here's a link to the site: