Some popular sites for downloading Xbox 360 BIOS include:
You don’t need a “BIOS” for Xbox 360 emulation or repair. Use Xenia for emulating games legally with your own disc dumps, and for console fixes, look into NAND flashing – but never download random system files from the web. Xbox 360 Bios Download
Many websites claim to host "BIOS v3.2.4" or similar files for Xbox 360 emulation. In most cases, these are either: Some popular sites for downloading Xbox 360 BIOS
If you are serious about preservation or emulation, here is the general workflow: and for console fixes