Streichers, Midway, and Brownells all sell dunny mags for most guns. They are Rings (the blue gun address above takes you to their site), but you'll save a few dollars at the above stores.
Don't expect a perfect fit, they rattle a bit. I wound up making dummy ammo and using "real" mags for practice.
A...O...Way to go, Ohio...(Chrissy Hein)