You can pick up memory at many of the local resellers, so long as you know what RAM you want.
You can try Memory Express or Techtronics, but I got my RAM at Westworld Computers. MyMacDealer may also have the RAM in stock, and if you really want to you can always order it online at places like CanadaRAM.
The 15" AlPB will take DDR2700 (333Mhz) RAM. Westworld sells it for $269.00, I'm not sure on prices of the other retailers, as I find Westworld has great service.