They can be found just about anywhere your keyboard fingers and cursor want to go. As far as getting the lowest possible price, that would take some extensive searching. My local gun shop stocks them for $24.95 new, but I usually buy used-looks-like-new for $15-$18 depending. A lot of folks seem to go with CDNN, but you'll need to calll them to make sure what they offer is in stock. If it don't say Glock factory or OEM, then it's usually not a Glock mag.