Truth be told I don't remember where I got my 68k Quake files for Amiga. I know that I found both 68k Quake and GLQuake stuff, although I can't make GLQuake 68k run in WinUAE with QuarkTex. Payback demo works fine, just not OGL stuff.
Not that it's important, if I ever want to play Quake again, I'll do it PC style. (doubtful though, UT2k4 is bette :-D )