Sorry, I don't know where to find them. They seem to have stopped manufacturing it.
But it seems the only Slot-2 cart I know of is the EZ-Flash 4 at shoptemp.
Or if you don't mind having to use a DS lite for GBA, you can just get a Slot-1 card (preferably an AK2i or EZ-Flash 5) with the EZ-Flash 3-in-1 expansion (which only fits in the DS lite).
No.
When DS flash carts were still on the GBA slot phase of life the big three companies at the time (EZFlash, M3 and Supercard) all released cut down versions of their GBA slot stuff to allow people to play DS games without the extra expense of GBA hardware.
The Supercard rumble was Supercard's effort here, and unlike the other two never gained the ability to even play small games.
DS ROMs and DS homebrew are then going to be all this is good for, and its DS ROM support ended some time in 2009 (probably one of the last to go but still 2009 so anything much later than that will likely never work). DS homebrew is probably not going to be much use either -- at one point it might have had a nice DS ROM dumper but today most will probably use a hacked DSi or 3ds instead (not to mention it might technically fail on some later ROMs, though not in a way you are likely to care about).
Short version they are pretty much useless which is why you presumably found one for cheap all these years later and are asking this question.
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