Welcome to R2R. There's a really active reefing scene in Denver and along the Front Range in general, and plenty of good saltwater stores. Also make sure to check out: https://www.marinecolorado.org/
I'll definitely have to check MASC out! Definitely a pretty active scene out here. Aquatic Art in the highlands has been my go-to place. The owner Chris is super nice and helpful with any questions I may have.