Nintendo Brand GameCube memory cards
I strongly recommend the Nintendo brand GameCube memory cards. Their quality is definitely superior, and you don't want to risk losing your saved games. I particularly recommend the Nintendo GameCube Memory Card 1019 for its higher capacity:
Mad Catz memory cards for the GameCube
Mad Catz sells a lot of cheap products. That's good. Unfortunately, some of their products are also very cheaply made. Mad Catz memory cards for the GameCube seem to generate a lot of complaints, and I can't recommend them. Nonetheless, if you want to take a chance on them, here are links to several Mad Catz memory cards that are available for the GameCube: