I was just looking for the F-ZERO: Legend of Falcon DVDs (ones with English subtitles) and noticed Amazon (US) are selling F-ZERO GP Legend (Game - US Version) at a really cheap price for less than $2!
www.amazon.com/F-Zero-Gp-Legend-Game-Boy-Advance/dp/B00022GIYI
Amazon do send items internationally if you live outside of the US (posted is a about $10 -$14 however), so in my case, the game would only cost me £10 in total. That is way cheaper than the ones I've seen on ebay and the item will be new as well. 
So yeah, if you're looking for a cheap copy, try www.amazon.com (US).