Disclaimer: This post is for educational and historical discussion. Downloading copyrighted BIOS or game files you do not own is legally murky. Always respect active publishers and developers. First, a semantic note. In the retro community, "ROM" (Read-Only Memory) typically refers to cartridge dumps. The original Xbox used a standard DVD-ROM drive and a proprietary hard drive format. So, when people search for "Xbox ROMs," they are usually looking for ISO files (disc images) or HDD ready folders (extracted game files).
But today, the original Xbox faces a unique problem: roms for original xbox
Buy a broken 1.0 or 1.1 revision Xbox, fix the clock cap, softmod it, and rip your own discs. If you cannot find the disc because the game is $400 on eBay... well, that is a conversation between you and your own archival ethics. Disclaimer: This post is for educational and historical