There are plenty of carts that work with the DSi, the best being either the AceKard 2i or the EZ Flash Vi. Stay away from crap like the R4i and DSTTi.
anyway the problem right now is because the Japanese release and the American release are so staggered some people believe that although these carts work on the Japanese DSi, Nintendo may have changed things around so that they won't work on the American DSi. This is merely speculation, I for one don't believe it.