Many people you hire from say "Odesk" will show they are independent contractors. However many of them will also work for development firm too. I suggest you find a solid 4 rated firm to go over your basic concept. Don't worry about the NCND because you want to protect your idea. These firms see hundreds of design concepts a day and odds are pretty good you aren't creating the next "Angry Birds". That said... you don't want to explain every aspect and details of your app in your Job Description on Odesk for the contractor/development firm. Once you find a few inquiries about getting your app developed, you want to get on Skype with the person you feel might be good fit for you. Make sure you feel comfortable on how you communicate with them. That's really important. There is a huge difference hiring a $30-$50 coder or a $10 one. The $10 one might be cheap, but will take 3-4 hours to do what $30-$50 coder can do in one hour for you. Make sure you confirm your code will be secured third party password protected service too. If you have been working on this concept for 6 months, that tells me you have put a lot of thought in it. Don't give away too many details, but find someone that understands your direction when you explain it them. Trust your gut instincts, but don't forget to verify past projects and experience before hiring them.
I hope that helps.
Remember... be a servant,