Gamestop :) Just kidding. Well search online. Also Wal Mart sometimes WalMart will have them. I remember seeing one there 2 years ago and they r no longer part of the top consoles for about 6 years now so it may be a bit tough :P Look at Craigslist and Ebay
Well I would suggest looking on a trustworthy website like amazon they have plenty of gamecube games for sale ;)
Amazons, thrift stores, and swap meets are the best ways to fing them.
I would say the best places are probably online like eBay.
Flea markets, rummage sales, garage/yard sales.
craigslist. Any online gaming store.
pawn shop cheap