I've had great success with all of these vendors.
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Another recommendation for Glockparts.com
Fast service. Also, they offer $3.00 USPS 1st class shipping for small parts.
A few brick-and-mortar stores around here stock Glock parts, but at inflated prices, so I use Glockmeister, Brownell's, or Midway. Prices are all about the same, so I buy from whoever has what I need.
Most Glock parts are so cheap that shipping costs as much or more than the parts themselves. The trick is to buy 3-5 of whatever you need to keep the per-unit cost down. It also doesn't hurt to grab several each of the springs and other little parts when you get some extra cash. I spent $50-75 here and there over several months and built up a righteous little stash of parts. Now I have everything I'll need to keep my Glocks running for years, and I can respring/upgrade an old police trade-in in ten minutes if I stumble into one.
Okie John
Glockmeister.
I've had good luck with Rockyourglock, they're local for me, always treated me fair
http://www.rockyourglock.com/
One tip on prices. If you have a C&R license (FFL-03), you can get some nice discounts at Brownells and Midway. If you don't get into buying the old guns, what you save will more than pay for the $30 cost for three years.