www.ebgames.com has been awesome for finding used GBA games. And they run 20%-25% off sales a lot, so shipping becomes essentially free.
(note: I do not work for ebgames.com. However, they won my final vote of approval when I ordered 2 games yesterday at about 1:00 and paid for 3 day shipping and they showed up today. That's quality. Mind you it's 200+ miles away, too.)