Well, first off,
RGB Classic Games has all the official releases of the shareware episode here:
http://www.classicdosgames.com/game/Doom.html
In addition, there are links to the leaked beta versions of the shareware episode between v1.2 and v1.9 in
this post, kindly provided by Litude.
As for the id Software FTP mirrors, here are some:
ftp://archives.gamers.org/pub/idgames/idstuff/doom/
http://www.doomworld.com/idgames/ind...=idstuff/doom/