TheMonkofDestiny: Ultimate Doom and Doom II + Master Levels are sold here on GOG. If you're looking for anything that aren't those versions, I don't think they're sold digitally. You might try your luck with eBay for the original disks/etc. but just expect to pay significantly more for those. I know John Romero auctioned his originals off through there not long ago but I imagine others with copies are trying to flip them for a quick buck now that the retro boom is in swing.
As for non-BFG Doom 3, read through the Doom series forum here on GOG. There's a thread in there where someone has uploaded the pk3 files for the original version and the expansion so they can be played in a source port. Definitely not legal but since the thread is still up, telling you to look for it shouldn't be troubling.
1. TBF I would always play the originals.....cutting out stuff to sell it in germany or to appease the red cross never sat well with me.
2. Eh, if one owns the originals or even the BFG I see no harm in it.