Let's take an example of Coinbase or Luno, who are their suppliers of bitcoins to be distributed to the buyers?
closed as too broad by Adam, alcio, Andrew Chow♦ Apr 23 '18 at 2:56
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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.