How many do you want to carry? A couple of the BlackHawk vests come with a pouch that carries the shells. Or if you have molle on your vest you could buy the pouch from them and then mount it on your vest.
I'd look into picking up a molle mag dump. Works good for shotgun shells and most have a drawstring or velcro on top to keep them secure if you're running ect. I have two. 1 to use what it's intended for and 1 for shells if i'm carrying a shotty. You can attach these to most tac vests that are molle compatible.