Few suggestions:
A) Slayer's Auto Install CD. It has original files on it if you need them. I never used this thing, but it has them. In other words: I dunno where you get it.
B) Do you XBox Live? If you do, then you'll need the stock dashboard... but if not, then you don't need it! I don't use the stock files anymore. I use UnleashX, xToolbox, and XBox Media Center. All of those are better anyway.
C) Mod a friend friends box! That's what I did when I still used the stock files. Then, use your computer to FTP. Then, just copy his stock files and use them.
D) You might be able to find the files on XBins. I dunno: I haven't been on XBins in a long time. I probably won't get on again until the next bios is released.
Good luck!
- Greg