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 wonder why you never found an answer....
They mine their own and get the rest from buying peoples bitcoin that is why they are called an exchange so they can exchange. That really was a stupid question I mean honestly.