I use CDNN to get my Glock factory mags. They almost always have the best prices on the web, and if you wait for a weekend to order, they usually run a weekend shipping special (on and off for the last few months, the special has been $5.99 shipping, and one or two free items like adhesive target stickers or gun lube). At this price, I can order one mag, delivered, and it's five to ten bucks cheaper than I can buy it locally.
Just make sure it says "new factory" if you want factory Glock mags. Be aware that the mags are purchased in bulk and re-packaged, so the mags won't come in Glock packaging. They also carry cheap imports and knock-off mags, but I don't buy or use those. Once in a while, they have great deals on used factory mags (police trade-ins, usually), but I haven't seen many of those lately. The used factory mags are great for "range mags", or general target/competition shooting, but I'm not sure I'd trust them for serious defensive use without extensive testing.