You can download original DungeonMaster by FTL from the DM&CSB-Encyclopaedia (see the "quick links" bar above) for MS-DOS-PC (or Amiga or Atari ST, usable on PC with emulator), or you use the Atari-version Paul R. Stevens ported to PC, CSBWin (usable in Win or Linux), which you also find at the Encyclopaedia, or you use the version George Gilbert completely programmed for Windows (RTC - Return to Chaos, dlable at
http://www.ragingmole.com).
Since this game is very old, you'll probably won't find it anywhere except ebay (where I bought mine sometime ago too).
Parting is all we know from Heaven, and all we need of hell.