I have DOOM 1.9 [the shareware version] on a Windows 95 computer and I just beat Knee-Deep In The Dead. I know of a couple places to get other .wad files for DOOM, and I was wondering if all .wad files are compatible with all versions of DOOM, or would I need to get special ones, and if so, where can I get them?
I did a bit of custom WAD writing back in the day, and from what I remember there were only two big rules when creating your own custom WADs:
So, if the WADs were written by anyone who followed iD's rules, you won't be able to use them with your shareware version. But, come on, you can pick up the full version of DOOM or DOOM II for pennies these days (either new on Steam, or for about the same price second hand elsewhere).
Of course now that the DOOM engine has been released for free under the GPL you could theoretically run any WAD with that, but if the WAD you download is a pWAD (patch WAD) that attaches to the (still iD copyrighted) iWAD files as a patch (as many of them do) then you will need to buy a legitimate copy of the iD WAD to use with whichever DOOM engine you have. However for total conversion iWADs that replace all graphics and sounds and don't use any iD copyrighted material (like the original Star Wars DOOM or Aliens DDOM) then you could use any of the free DOOM engines available now.
AFAIK you cannot use the Doom 2 or Final Doom wads with Doom 1.9. Also, given that you have the shareware, third-party wadfiles won't work either.
I strongly recommend getting either and then using the Doomsday Engine instead of the classic Doom engine. Its cross platform and has lots of new perks.
Also, Doom is not abandonware, so its illegal to download the WADs without owning the game.