Answer:
Game cards can be bought at any game store.
What games are you interested in? Some games offer their own game cards (like Blizzard for WoW), some games accept ultimate game cards or prepaid card, which can be used for many online games and even Facebook's Farmville and so on (for example Paysafecard, which is my favorite).
When you know what kind of a card you want, just check with their homepage and find the nearest retailer.