I’m sure he thought he had ethical drug dealers, why would they sell Prince bad drugs?
They didn't do it on purpose, but could not have tested the drugs for Fentanyl, which ethical drug dealers would do, be it by way of an official testing kit, or from personal use.
You'd think Prince would get such a diligent/responsible "drug dealer to the stars", but I guess not. Or maybe he was in a pinch, and called some secondary dealer he didn't trust as well.