Blackhawk polymer double pouch. You can clip it on, or run a belt thru it. Has tension pads inside pouches to hold mags. Academy has them, at least they do in my area. Good stuff, trust me.
For range and other OC-friendly situations, I really like the Uncle Mike's Kydex double mag, single-stack pouches. I don't know How they work, but they work really well. They don't come out unless you pull it out and it won't wear out or fray like leather or nylon.