A majority of them get their BTC's from users selling to them, They offer it in 2 ways "i.e buying and selling", I guess it's the same BTC but they just add there commission to the price, I don't think there is a specific way to get their BTC's other than from the users selling.
I can be told you for took one example about the exchange site like bittrex and every a new sign of the coin are sue for paying 3 btc with them. and this is also running in the another place for donating some of the coins for paying the exchange sites.