How does crypto exchange got supply of bitcoins? Let's take an example of Coinbase or Luno, who are their suppliers of bitcoins to be distributed to the buyers?
You have to think about it simply, its exchange service lets consider that they're exchanging potatoes for tomato instead of Bitcoin for money.

you've bought 50 kilos of potatoes from a farmer, a customer gave you 2 kilos of tomato for 1 kilo of potatoes and go on, the farmer here is mining pools which is the main source of bitcoins. 

So I would say Mining pools and Bitcoin sellers, but there's no specific answer to that question so I've marked it as Primary opinion based.