Supercard DSTWO can emulate GBA games.
And if you're familiar with emulation, you know that it's not very ideal.
But it's the only option if you own a DSi.

EZ-Flash on the other hand runs GBA games, without emulation.
That means that they work as they would on a GBA but since it's a slot-II card you can only use it on DS Lite or older.
I got mine for 20$ I believe but that site does not sell them anymore.