I would strongly advise against GBA slot supercards. Their compatibility and list of issues is long and never likely to be resolved where everything else that is not a clone is fairly sorted.
Other than that if you can't find a 3 in 1 or something similar (shop01 has them for £12.49 at this point, though will need a case mod to work on anything other than a DS lite and will obviously need a DS slot cart to manage them) then time to save up a bit more.