I don't know if there exist any other commercial store for Quest/Go than the Oculus Store, but for experimentations/betas, SideQuest is an example of how to play things that don't come from the store (https://sidequestvr.com/).

Sidequest currently requires a developer account which requires signing an NDA with Oculus/Facebook. It is basically sliding through the cracks at their mercy until they expand things (which they have announced but haven't fully detailed; they did say sideloaded apps will still go through a store approval censorship process, and I'm not sure about monetary cut).