When you visit a Gamestop that sells GBC games, remember to ask if they have any others in boxes or stored in the store somewhere. I went to one in Wilmington, Delaware that only had about 10 in a bin, but I decided to ask. The guy looked under the counter and pulled out a huge bag of GB/GBC games. I went through and picked out a ton of games I've been wanting to find. I couldn't believe these games had just been sitting under the counter for months.
Just to name a few:
Pokemon Yellow, Crystal, Blue, and Silver
Link's Awakening DX ad Oracle of Ages
Lufia
Megaman Xtreme, Xtreme 2, III, IV
Game and Watch Gallery 1, 2, 3 and Super Mario Land 1, 2, 3