buy bitcoins – How do they sell cryptocurrency websites with fiduciary payments?

I am trying to create a platform to sell cryptocurrencies in my country. The way it works is that the user enters my website, selects a currency that he wants to buy, then the amount he wants to buy, pays in our common currency (for example, USD) and we send him the cryptocurrency he has. I buy. The procedure is like most crypto stores, but the question is, how should I get crypts to sell? Let's say that a user wants to buy 5 Bitcoins, how should he have 5 Bitcoins to give him? How do other websites do this?
Any help would be appreciated.

Thanks in advance.