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?
Simple. They will get from miners. All the mining companies need to use bitcoins for the purpose of needing funds to maintain their mining farm, electricity cost would be one of biggest expense for them. Other than this Coinbase will be able to use/circulate wallet holders bitcoin similar to exchanges.