The best hiring tool you can use is your personal network. Connections, connections of connections, etc. Twitter is perfect tool for being in touch with first class developers (at least in my country), then there are blogs, developer meet-ups, conferences, workshops. Go everywhere you can build a personal relationship. And after that, if all personal ways fail, you can still start looking around on LinkedIn.
Find an app that you think is good quality, and shares something with what you want a developer for, then use the support email link on their App Store page. Or search for their website and an email link there.