AZarmament is a great source for Magpul magazines. Like everyone else, they sell out fast. But they seem to get them in regularly.
I called three weeks ago about 308 magazines, and they told me to call back in a week. I did, and they had a few in stock. Today I decided I could use a few more so I called again, and they had a few more in stock. They charge retail, which in this market is fantastic. They also have some really nice AR accessories.
In total I have ordered from them 3 times, and they always shipped promptly.