The way I see it, you have four options:
1) Wait for Nintendo to release its GBA official emulator/port through the Eshop (they have some for early adopters, but none for the general public at this time).
2) Install a GBA emulator on a portable device, maybe get a blutooth controler like a PS3 remote or a Phonejoy to go with it.
3) Buy an actual GBA system for a lot cheaper than a DS2 or any other flashcard. [1]
[1]
http://www.ebay.com/itm/GAME-BOY-ADVANCE-with-games-/331112328974?pt=Video_Games&hash=item4d17d52f0e
[2]