You may not be familiar with Hawkepaks but you should be. They offer some great bags and tactical gear at very reasonable prices. I was recently looking for a way to carry shotgun shells on my current gear without having to buy shotgun specific gear and I came across the 15 Round Shotshell Strip from Hawkepaks.
These shotgun shell carriers are designed to hold 15 shells in a double stack that allows them to fit in a standard double M4 magazine pouch. They retain the shells via elastic loops and each carrier has a grommet that can be used to lash it to the magazine pouch for retention.
The 15 Round Shotshell Strip cost just $9.99 and are made in the USA. Check them out at Hawkepaks.