Hmm, i think all the cards which can work on the normal 3ds can also work on the 3DS XL, such as this acekard 2i: http://www.dsflashcart.com/acekard-2i-flashcart-for-nintendo-ds-dsi-3ds-p-251.html or the DSTwo and r4i gold 3ds. those are top 3 flashcarts.
of course Supercard dstwo is the best flashcart, http://www.dsflashcart.com/supercard-dstwo-best-flashcard-for-ds-dsi-3ds-p-252.html if you want to play gba games on your 3ds, dstwo is the only choose, it also has some other features such as real time save, real time menu..
