Memory card for PS2

How much is a memory card for a PS2? (It's amazing one isn't included with the PS2 when you buy it.)

Are there different kinds? makes? brands? sizes? Are they different prices? What's the minimum/best quality needed?

Can you save more than one game on a card or do you need a seperate card for each game?

just got a ps2 and need a memory card. had a look on [broken link removed] and basically not too sure if all cards are the same, some say slim and others don't. also roughly which size card is best? Cheers!