Buying Unlocked iPhone from Apple Store for Boost Mobile?

I want to update my iPhone and currently use Boost Mobile. I have always gotten my phones directly from Boost, but noticed that they are lacking inventory and newer models. Does anyone have experience with buying a "unlocked" phone from the Apple Store directly and using it for Boost? Do you need to obtain a SIM card from Boost? I would love to hear the process.

Thank you!