This is what he said about writable DS cards:
I got mine from: http://www.r4card.com.au/ The one I got was the R4 SDHC Card for DS / DS Lite + 4GB Micro SD Memory CardHe points out that he's not endorsing this website as the best or only place to buy them, just that he bought one there and it worked. The teensy little cards slot into an R4 cartridge which is supplied and works in the DS itself.
Gotchas: Make sure to get the one which says DS Lite if that's what you have (the DS is different, I think). The SD card is teensy (little-fingernail sized), but you probably know more about that than I do. It comes with a USB-stick-like reader for you to copy stuff onto from your PC. The big gotcha that took me hours to hunt down was finding the right OS to put on the card. The trick was to use the URL printed on the box the card came in (rather than just googling for generic r4 card OS's). D'oh!
Download the OS and copy the files onto the SD card and you're set. Copy your downloaded games/music/pictures into the top directory on the SD card and the OS can find them.
I also had uTorrent (micro-torrent?) recommended to me - seems to work much better than other torrent downloaders we've tried. Can probably dig up places for you to look for games too.
Merry Christmas everyone!