There are several velcro side saddle 12 ga caddies out there and they are not all created equal, most are crap. I am only aware of 2 out there that are what I would consider recommending and what I've use/used. The first is made by a company that makes gear for competitive shotgun/3 gun events. The name eludes me at the moment but if I can recall/find it I'll post it.
What I highly recommend is SOE's 12ga micro rig, it holds shotgun shells but can also hold M4 magazines if you don't need shotgun shells for a specific load out. If the rig itself is more than you want, they also sell 12ga shotshell pull out trays/pouches that can be used in your typical magazine pouch or in one they make that allows for one in the pouch and one on the outside. This gives you the ability to carry 24ea 12ga shotgun shells in the space two M4 mag pouches occupies. The flexibility of the shell cards is that is allows you to carry one on the side of the shotgun if you want and once its empty you can peel it off and put another one on off of your kit so you always have something on the gun. Check out both products here.
12ga micro rig
12ga pull out trays
The great thing about SOE is if what you see doesn't fit your bill, they will modify /make the item you need.
Edit: the link to the first caddy I mentioned is made by 3gungear.com the peel off version can be found
HERE