They work on all DS's (DS lite, DSI, DSI XL, and 3DS), I get them from goldenbridge in hongkong, so it takes a while to arrive, but I know sinno uses someone within europe, I am sure he will reply with where.
When you get your card, at that stage either Sinno or I will point you to the latest firmware at that moment, they generally come with firmware on, but sometimes it is not the latest.
Code:
http://www.eachgame.hk/search.php?category=0&keywords=acekard+2i
this page shows the different size SD cards and the prices of them. Where are you in the world, just so we can point you to the best site for your region, (country will do, don't want your postal adress or anything lol)
Sorry, just had a look around through the DS section and found one of the places that Sinno recommends, it is the address below. you should probably compare card sizes and prices.
Code:
http://lightake.com/detail.do/sku.Acekard_2i_AK2i_1.4.1_Compatible_for_3DS_NDS_Lite_NDSi_NDSi_XL-36778