to me the main source from where the exchanges get their money is from the transaction fee, may be they also have invested their bitcoins in different places,
Exchanges when established needs capital and they buy volumes of bitcoin. But later on they dont need to buy bitcoin they just earn it from the transaction fees and other fees they charged during trading. And also they charged the developers of any coin with some amount before they could enter in their site. So exchange is like a market that charges the vendors.