Only buy from reputable dealers like MidwayUSA.
Personally, on magazines like those in a Glock, the manufacturer original is best in many cases. I shy away from replacements like Mec-Gar and such. They may operate “just as well” but I don’t have to take that chance.
Brownell's and Natchez are good too. Haven't had any bad experiences with CheaperThanDirt either.