GBA:Shell
Replacing the plastic shell on a Game Boy requires a tri-point screwdriver to remove the factory screws. Some replacement shells include replacement screws that are Philips instead of Tri-Point, and may be shorter or longer depending on the manufacturing process. If your replacement shell comes with screws make sure to use those instead of the factory screws, as installing screws that are too long could crack the plastic and screws too short wouldn't be able to provide a secure fit.
Replacement shells can come pre-cut for replacement screens. Some third party displays (particularly IPS screens) are larger than the factory screen meaning that the plastic surrounding the screen cut out needs to be trimmed to allow a larger opening. If the shell you have is not pre-cut and you are using a larger screen, it needs to be trimmed using flush cutters and then filed down.
You will also need to install a new screen lens if your shell doesn't come with one included. Depending on if you are upgrading to an IPS screen you can just swap the one on the factory shell to the replacement or buy a new one to install.
Game Boy Advance (AGB)
Where to buy:
BoxyPixel (BoxyPixel machined aluminum IPS ready shell)
ExtremeRate (ExtremeRate IPS ready shell)
ExtremeRate (ExtremeRate replacement shell)
FunnyPlaying (ITA/TFT ready shell, only fits FunnyPlaying's Laminated ITA/TFT screen)
HandHeldLegend (RetroSix IPS ready shell)
HandHeldLegend (RetroSix IPS ready shell, same as above but includes the "Nintendo" logo above the cutout for the screen)
RetroGameRepairShop (FunnyPlaying IPS ready shell)
RetroGameRepairShop (FunnyPlaying clear mirror polished IPS ready shell)
RetroGameRepairShop (unbranded IPS ready shell)
RetroGameRepairShop (unbranded replacement shell, some are IPS ready depending on color choice)
RetroGameRepairShop (unbranded replacement shell)
RetroModding (FunnyPlaying IPS ready shell)
Game Boy Advance SP (AGS 001/101)
Where to buy:
ExtremeRate (ExtremeRate IPS ready shell)
HandHeldLegend (unbranded IPS ready shell)
HandHeldLegend (RetroSix IPS ready shell)
RetroGameRepairShop (unbranded IPS ready shell)
RetroGameRepairShop (unbranded replacement shell)
RetroGameRepairShop (unbranded Pokemon replacement shell)
RetroGameRepairShop (unbranded limited edition variant replacement shell)
Game Boy Micro
Where to buy:
RetroGameRepairShop (unbranded replacement shell)
RetroGameRepairShop (unbranded replacement faceplate)
RetroModding (unbranded replacement faceplate)