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.
To open an exchange you needs lots of bitcoin, i think that most of the popular/biggest exchanges are early adopters of bitcoin. So they have a good amount of bitcoin, otherwise they have invested a large amount of money buying bitcoin in the past and also another source is the profit from every exchange that is made there which is at least from 0.2% to 1%.