This is the best GBA light I've used or owned:

Outside of modding with the afterburner or getting an sp I'd say it's your best option. I'd also used a worm light, terrible. And I believe the halo light? If I remember correctly it did a spotlight effect so I had to hold the gba at a very awkward angle. The flood light by gamester gives an even layer of white light with little to no glare. Very bright to, and it uses a white light so it's better for your eyes than the cheap yellow light ones. Plus, it uses its own AA rather than eating all of the gba's juice. I think it's like ten bux. When I'd purchased mine, the package had batteries and a little demo button to test it for yourself. Check it out!
*Edit: Links!*
Manufacturers site:
http://www.gamesterusa.com/gbafloodlight.htmlI think I'd purchased mine at Gamestop here in WA. I've seen it at Fred Meyers as well. My brother in NE picked his up at the Furniture Mart (the mart,,, kinda like incredible universe / bestbuy)
Hope that helps some!
Of course, if you're going to be using it a lot. Keep in mind all costs and factors:
Using origional gba: needs replacement batteries for gba/light (assuming your not using rechargables. Extra hardware to cary around. Added cost -> Initially 20$, + 5$ batteries per trip, assuming you keep a fresh pair handy.
Using gba:sp: 79.99 up front (if you sell the old one, do it on ebay. trade in would be worthless). No added money per trip since it has a built in recharger. More compact. About half the size and no added hardware.
Personally here's what I'd do:
1-2 trips, no general use, get the light and call it good.
3+ trips and other general use, get the sp. A lot of money now, but it'll save money in the long run and space when packing.