The best place to look for someone is in your personal network, ask around among your friends and colleagues if they know anyone with an Industrial Design or Mechanical Engineering background. You are sure to find someone, there are more of us about than you think, take them out for a beer and have a chat about the idea.
A word of advice though, be upfront with the idea, don't try and suggest meeting up without disclosing the idea beforehand. Just as software developers are used to people pitched them their app idea, product designers are used to having people pitch them ideas too!
Failing that, there are a lot of "Product Design Engineering" firms/consultancies that do this every day, I used to work for one (I'm a former Product Design Engineer, now Software Developer). Most of these firms are multi-disciplinary, they have Industrial Designers, Mechanical Engineers, Electrical Engineers, Software Developers all in house. They also tend to have a network of other disciplines that will regularly work with (optical designers or material specialists for example). They will be happy to give you and hour of their time to hear about the idea. But be aware, going that route will not be cheap once you move past an initial consultation.