Depending on what's available in your area, I would also recommend Gamestop. To get an idea of what games to recommend, what was your last purchasefor PS2?
Its difficult to give a definitive answer since different games will use different amounts of blcoks to save information. But if you 've only used up about half of your card then, you've still got what, 125 blocks remaining? No need to run out and buy one just yet.