If you own a modified Xbox 360 (JTAG or RGH modded), you can dump the NAND (the internal memory) yourself using tools like Nandpro or Xell . This creates a perfect, legal backup of your console's firmware, which you can then use with Xenia to play the games you own.
(the closest thing to a BIOS file)
The BIOS is copyrighted software owned by Microsoft. Just like downloading a game ROM you don't own, downloading a BIOS file is a violation of copyright law. bios xbox 360