Which flashcard do I need and where can I buy one in the Netherlands

Hi,

So I'm a bit of a noob, but I would like to download games for my recently bought DSI XL.

I'm not sure which card I need and where I can buy/order one.
From what I've read the R4 cards are great, but you have to watch out when ordering one, because a lot of them are non-working copies.

So, yeah. I was hoping someone could help me out.