Hey man, I work at Electronics Boutique and I must honestly say that I have yet to see a fully decent light for your GBA's screen. A lot of the ones I see illuminate the edges but leave a slight dark spot at the centre of the screen or are far too bright. There are definately good ones out there but I have yet to come across one.
A few of my friends traded in their GBA's for GBASP's just so they could get a decent backlight, I however would recommend very highly that you do NOT do that, since your GBA was a gift
Hopefully someone else here at the TFP will be able to help, the GBA is an awesome system and feels much better to handle than an SP.