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.

GLOCK Mags

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.

Thanks DJ! All the older threads on the subject are outdated it seems.

I'll be sure to check them out this weekend!