A good cheap one with expandable memory that will fit in a GBA Player is the ORIGINAL EZ 4 (Mini SD), not the deluxe,compact, mini versions. You can find one at DealExtreme. 2 more would be the M3 Perfect (a bit more pricey) and the Super Card (Less compatability). If you use the M3/SC, get either the SD or Mini SD versions. Not the micro/lite (rumble) versions for the GBA Player. If not, you can try the NAND cart which include the EFA and eLink. They have almost perfect compatibility also but take a while to write to.