smaller game stores in your area are your only retail bet, but aside from that, I can only see you finding a copy on Ebay or Half.com...
I got damn lucky and a CAG hooked me up with a sealed copy a few months back. There's a FAQ on CAG somewhere that tells you how to spot a GBA bootleg... prolly in the Nintendo section.
Anyway, good luck on finding it. It's an awesome RPG and definitely worth whatever you end up paying for it.