The Magic Box has a list of Japanese sales, and GBA is still doing fine!
This list is actually dated at July 2nd-8th, but it's still nice to see what has sold in Japan.
1: Yugioh Duel Monsters 5: Expert I
221,654 units sold (new)
3: Breath of Fire
22,236l units sold (new)
10: Tactics Orge Gaiden: The Knight of Lodis
13,466 units sold (200,741 Total)
11: Super Mario Advance
10,260 units sold (614,625 Total)
26: Choro Q Advance
4,489 units sold (12,499 Total)
30: Pawapuro Kum Pocket 3
3,906 units sold (192,210 Total)
Meanwhile, 94,380 GBA consoles were sold, more than any other console.