Two that come to mind are Coinbase & Circle. I did a quick google search, possibly wrong terms. The information I got was useless. I am sure this has been discussed. If there is a good article/link on this it would be appreciated.
I think exchanger obtain bitcoin from fee, transaction fee and the withdraw fee. we know if they are millions of users then it becomes enormous profits, not to mention some of the exchangers often plays a coin prices. Thank you