As far as I know, the Supercard DSTwo should work with all models of DSi, including the XL (at least that's what their website says). I have a regular DSi (not sure of the firmware but I can check when I get home) that I regularly use it on.
The only downside is that it's becoming increasingly harder to find, but I would say it's definitely worth the effort if you can find one. I know it's one of the few (if not the only?) slot-1 cards capable of GBA emulation, which was a big sell for me.