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.
Most of the exchanges platform get their bitcoin to all users who sign up/register to their exchange sites to trade their coins through in every transaction charge fees that there gonna make whether they are going to buy a coins or withdraw their coins.