I have an Advantage kit for the G21. Bought it off gunrbroker.com auction site from a dealer. Probably paid about $25 too much but got it right away.
If you're looking for one right away, suggest you look on one of the auction sites as they seem to be available for a small premium.
As for the kit, it works fine--about as accurate as the G21, and completely reliable. Mags are available and reasonably priced--$25 from Advantage.
The only drawback is that the kit is so lightweight (aluminum slide) that when mounted on the plastic frame, the combination feels like a toy pistol, and not a very good simulation at all of the real thing, at least in terms of weight. If you can get by that, its a fine piece of equipment.