Afterburners pop up on E-Bay from time to time. That's the only source I know of. Sometimes they go for around $50-$60 - but as with all things on E-Bay, you can get a better deal if the timing is right.The good news is that you can buy Afterburner Cases for cheap, so you don't have to deal with cutting the front half of the GBA to make room for the light guide (the hardest part of the installation IMO).
I have seen the Halo things you're talking about, I think they're junk. Plus they have to connect externally to the link port for power, so they make a bulge in the casing. I don't think it even has a light guide, let alone anti-reflective film like the Afterburner.
I had my afterburner GBA for a couple years and I loved it. I didn't want to move to an SP because I preferred the larger form factor and the ability to buy batteries off-the-shelf as needed. Then I got an afterburner for my girlfriend's GBA, but because I was rushed and overconfident from having installed the mine without mishap I totally botched hers, so I let her keep my GBA and got an SP for myself. It's really quite nice and I've grown quite attached to it. Now and then I miss the full-size shoulder buttons (playing Street Fighter, in particular) but it's good stuff.
I don't know if it'd be possible to install the SP screen onto a regular GBA... in principle, probably, it's the same size screen and all... but those fragile laminated ribbon cables absolutely suck. I wouldn't want to try it.
If you get an Afterburner, just be sure to take your time, keep your work area clean, and familiarize yourself thoroughly with the directions. It's not a hard installation at all.