Probably FunnyPlaying or RetroGameRepair (been awhile can't recall off-hand), it's the IPS v2 screen, which costs only like $45. And adding the USB charging component is like another $10. And that's really all you need to make it a great modern handheld, about $55-$60, but you can go on to further mod if you like, but no mod is outrageously priced at all.
Also you can get better products that will cost more money of course. When I did mine, the IPS v2 screen was the top of the line, and it still pretty much is, but now they make laminated displays so that there's no chance of any dust falling between the lens and screen (not that I have any, but a screen pressed directly against the lens will always be a better display). And there are even extra mods you can do that give the GBA AV out, and now even HDMI out. People have added bluetooth audio, made the L and R buttons more clickier for better feedback, changed the power LED color, and put LEDs in the GBA itself which can look great for systems that have a solid colored shell but with translucent buttons (mine being all translucent would not make it an ideal fit for that kind of mod). And I'm sure there are other clever mods I've omitted to mention here.
I suppose the point is, while Nintendo didn't ship out the GBA of our collective dreams, and were never going to, we now live in an age where we can pretty easily and affordably turn our old GBAs into our personal ideal versions we always wanted them to be.