I have one on my .308 I find it works quite well but I don't fill it to what they say I can. It allows me to comfortably load 5 rounds but the sixth and seventh to me seem a bit too tight. I can barely get the seventh in there so I stopped trying and only go with 5. I know the stock mag fits five but the fifth is really tight, now I can feed 5 in really easily, but 6 or 7.. I'd go as far to say that it is false advertising to say that they can load 7 without changing the mag spring.
I bought mine from the company. They have a website but it is buried down in google somewhere. Otherwise try brownells like Terry said.