Hi
Im developing a Bitcoin exchange as a way to learn Bitcoin development. I have a node running, and an integration with Stripe to acquire fiat (USD, GBP and EUR) from users. Users provide a Bitcoin address so there is no storing of their keys.

Im guessing the next part - were I to actually go live with it - would be to bulk buy Bitcoin, work out a conversion rate between the fiat and this Bitcoin, and dole this out to users as they buy?

Or is there some API service to buy Bitcoin on-the-fly?

Thanks