Go on websites like Odesk and find somebody suitable.
But trust me it is hard to find someone whos is decently. The hourly rate you should expect is ~35 usd.
Look for young guys (around 20 years old) who are studying something like EE or embedded systems
Do not get the "old" guy, as he might be good at his job he will not be as well balanced and as fast as an young allrounder. Look for USA/EU people not india or whatever
I'm still an Uni student but I do freelance work for multiple customers everywhere in the world.
Things that I don't like or might not have time for I just outsource myself. I.E. I have an russian guy with 10+ years professional pcb development and he costs less an hour than you might earn washing cars. He delivers excellent quality work.
Find someone that can do hardware + firmware + cad + prototyping + little bit of app/pc software development experience.
Because he is good at hard + software, average at CAD and prototyping and understands the basics of app/pc software development. The app/pc software development will ultimately done by someone else who is more experienced in this kind of stuff. But the EE guy will know if stuff will work or are just fucked from the beginning.
If you want feedback on design or want to know how much some development might cost and the time-frame than just contact me via pb or whatever. I can help steer you in the right direction.
For all the people saying do it yourself:
While I agree up to a certain point, you just can't do it all.
Time to market is essential in this day of age. While you might finally finish an prototype in a few months time, those months time also have an price tag. My advice is get a clear idea of what you want. Determine the hardware specs as good as you can, make a text document with a clear description of what the firmware has to do and sketch to the best of your abilities the case outline.
The freelancer that you will hire will take much less hours to develop your end product and it will be done right the first time.
It takes money to make money.
Expect around 1000-2000$ for an prototype (hardware, firmware, cad (case)).