I believe it isn't posted here because it isn't needed to run the emulators here. Could be wrong though, since I use one that runs without a BIOS.
"NintenDON'T sue me please haha"
I'm not sure why there isn't a download link for one on this site, but considering the PS2 BIOS is hosted here, I'm going to take the chance that it's okay to post a link to one for the PS1.
That links to an archive that contains multiple versions of the PS1 BIOS. Look for "ps-22a.7z" inside that archive; it should be the SCPH-1001 BIOS that's typically recommended.