That question has come up before and few good answers, if any, were posted. I am sure that where you might go has to do with how well far along you are - is it just an idea or ready for manufacturing. Often one of the most important things is money - can you pay for all of the resources, materials, etc. My brother is in the manufacturing business and is often approached to make something. Unless the person or company has a good, established credit rating or is paying up front he can't afford to take up time on the idea.
It is a good question though and there must be companies who provide the necessary service. I've sent some emails to friends in the electronics manufacturing business to see if they have any ideas.