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.
People who register accounts at these exchanges deposit bitcoins in order to
buy altcoins.This is how they get bitcoins,i guess.
If all the exchanges buy btc from Coinbase and Circle,then how Coinbase and Circle get their bitcoins?