You're probably going to be ordering online. Speed Shooters International (Cal based), Shooter's Connection and Dawson Precision are probably the biggest. If you're shooting production, most carry 4 or 5 mags on the belt and one in the gun, limited 3 on the belt and one in the gun. I always carry at least one more than stage demands....I've heard of magazines sometimes messing up and there might be the occasional dropped magazine