Ask HN: Best resources for hiring a contract engineer?

Our start up is looking to hire our first lead iOS engineer. We've done all of the development ourself up until now and now need someone with more expertise who has scaled an iOS app to 1 Million plus users. We would like to eventually hire them (lead iOS dev) full time if they are a great fit for what we need. What are some of the best resources for hiring a talented US Based Contractor?