I use the Fox tactical triple MP5 mag pouch. They fit great, and tight enough that you can tuck the flap in behind them with no worries they will fall out.
It runs 14.00 on amazon. I have one on a mini chest rig, and one with a shoulder strap. Throw the strap over your shoulder, and use a molle strap as a belt loop, makes a handy way to carry 3 mags.