The Glock Mags at Botach are the real deal.
@ $25 with 10% off right now and free shipping on everything they sell, it's a no brainer IF they happen to be in stock.
Botach is fine to deal with, customer service leaves a lot to be desired, but overall; in my experiences they have been excellent.
This... or generally anywhere I trust that I can pick them up in the less than $25 price range shipped and preferably as much less than that as I can get.
GTdist.com is another source that usually has them at good prices.
And don't buy anything but Glock OEM magazines. Glock mags are plentiful and cheap, buy a bunch of them. There is no need to buy aftermarket Glock mags and put up with that frustration.