You can find plenty of writers on the middle agency sites like eLance and the like. Reddit also has HireaWriter sub. You can also vet the writer with a simple Skype conversation. Some of the few clients I've pulled in have asked for a Skype conversation to verify my native English status.
Like Major said, price is a missing key component to answering you question. With that said, I've seen American writers work for as low as $9 per page on some of the company sites. I've reluctantly worked for that little in the past.