Brownells, Glockparts, and Top Gun Supply. All are excellent, ship quickly, have decent stock, and are inexpensive for shipping. I've never ordered from Glockmeister, but heard good things. Midway is also very good if you're combining orders due to shipping costs from UPS.
There are some good choices out there, also Glock themselves: http://us.glock.com/documents/GLOCK_..._Certified.pdf or http://us.glock.com/documents/GLOCK_..._Certified.pdf if you are certified as an armorer by Glock. (Kinda fun thing to do if there's a GSSF match in town).