Noticed others mentioned the distributors but i wanted to mention i just purchased some composite Pro Mag magazines,if they manufacture them for your gun i find them to be of excellent quality.
Pro-Mag does seem to have a good reputation. The one brand which I personally would stay away from is U.S.A. They look cheap and are of poor quality. Also malfunction in a gun which never jams with any other mag. I bought two of them during the ban as they were the only ones I could find.
After I got some good quality ones, I literally took a hammer to the U.S.A. ones.