There is almost no chance you will find one of these. I got one on ebay years and years ago and it seemed there was one guy selling them and he had only 3 left. I have never seen one again anywhere. I think you will have to be relegated to using the halo.
Ended up getting a Halo. It's better than no light, but it really sucks compared to the Afterburner. I love the original GBA's form factor (and the fact that it runs on AAs), but the lack of a lit screen is a real killer. Beginning to wish I'd never sold my old one... but I was pretty poor at the time, and it was the only way I could afford the (then-new) DS.
We still need to get a second GBA. Not sure whether to get a second original GBA, or an SP. It's eyestrain vs. hand cramps. (Seriously: the GBA is possibly the most comfortable-to-hold of all Nintendo's handhelds. The SP is cramp central).