Glocks are
supposed to be sold as a "
Glock Pistol Set" (<--- clickable link) but not every retailer does that. Sometimes they'll take a magazine out for display, sell it with another pistol to a "buddy" etc.
I have three Glock magazines that are genuine Glock. I paid $30.00 with tax for the third magazine (my pistol came with two) locally but they can probably be had cheaper online. I've never tried an aftermarket magazine although after firing 230 rounds yesterday with only three 7 round magazines I may invest in one of those crazy long korean-made knock off mags just for firing range use.