I'm sorry if this is in the wrong section (or even the wrong topic), but can someone explain to me what's the best and cheapest supercard for NDS?

I think you are in the wrong section.

Well, I use a supercard (even though most of people here say it's illegal lol). I suggest you buy the MiniSD one, coz it can play GBA roms and DS roms. I don't know about the cheapest though. I'm using SC Compactflash, and it sucks.