The r/Roms Megathread is a standard resource for finding firmware and system files. How to "Flash" a BIOS Image
: Unlike a standard PC BIOS, the Xbox BIOS contains the system kernel in a compressed, encrypted form. BIOS File Sizes by Console Revision Flash Rom Image -bios- Xbox Download
While you can dump a retail BIOS from your own hardware, standard retail images have restricted DRM that prevents them from booting unofficial software or homebrew in an emulator. The r/Roms Megathread is a standard resource for
Compatibility & target mapping