That you have a reasonable chance of still finding? Ez flash 3in1 for slightly less than 30 dollars
You may be able to get an used GBA traditional (external programmer) flashcard for less, but they don't really fit the "40 games on microsd you can select from" model, or an early GBA compactflash/sd/minisd flashcard
Higher in rank are standalone slot-2 cards, from the Ez-IV ($40 new) to the Everdrive (triple digits)