Prepography affiliate Sportsman’s Guide has some full capacity magazines in stock for:

• AR-15′s ($26)
• AK-74′s
• AK-47′s (30 & 40 round)
• SKS’s
• PS90′s
• Many others

Brownells supply of Modern Sporting Rifle Magazines is pretty tapped out but they still have a number of full capacity pistol magazines.

1/25/13 Update:  This morning LuckyGunner posted that they have 2, Thermold 20 round AR-15 Magazines and 37, 30 Round Magpul AR-15 Magazines available for $20 and $44 respectively.