Rewarding users with airline miles

I am looking into developing an app that will reward users with airline miles. How do businesses like car rentals, hotels, RewardsNetwork, TheOpinionTerminal, and some other apps (MOGL or a defunct app MilesLife comes to mind) tap into airline miles as they need to?

Is there a broker that offers bulk buying capabilities with the intent of redistributing those miles to the customers? An API from any major US airline?