Wanstalls had a bunch about a year ago. Were about 30 bucks. I bought a couple however didn't much care for them. The spring is a shortened one that sits on the pin (5rd). I found it caused binding and feeding issues due to the spring modification. I tried a standard gi mag spring hoping that once the pin was reinserted, would alleviate the binding due to the longer length but the spring shape wouldn't fit in the body of the mag.
Hope that helps
They probably work fine without a rivet, but thanks to Mulrooney and the previous PC government we have to bastardize our mags. (I see gang bangers with pinned mags all the time)