As that is the case, may I recommend checking out reviews for GBA flash cards, in places like GBATemp.net, to determine which is most likely to suit your needs. Then it’d be best to do a full Google search on the chosen subject. Since I don’t think most flash card manufacturers still develop cards for the GBA, you may come across a lot of listings with no stock and no guarantee they’ll come back in, but be vigilant, and you’ll find one you want.
Beyond that, I don’t think we can help you. But when you do find a cart that suits your needs, and have any trouble, be sure to check the giant flash card topic.