The best "DS" Flashcart is the supercard DSTwo. its like 40$ but you wont regret it!. play GBA, snes, nes and ds backups on it. and it works on any 3ds as well.
for "3DS" Flashcart you can only have the Sky3ds, and (MAYBE Sky 3ds clones)
for "3DS" Flashcart you can only have the Sky3ds, and (MAYBE Sky 3ds clones)