1. For shareware, demos and free (including commercial turn free) games my first stop is DOS Games Archive and the second is RGB Classic Games - (the latter does not seem to open anymore).
2. Online distribution platforms like GoG, Steam or Zoom Platfrom are hit and miss. I have quite a few games from GoG that work in DOS, but overall it's a gamble.
- some come in just fine (e.g. Terminal Velocity, Tyrian 2000, Star Control or Star Control II)
- others come as preinstalled (and sometimes patched) installation with full CD image available (e.g. Quake and mission disks, Strike Commander, Lands of Lore, Alone in the Dark 2 or Alone in the Dark 3)
- then there are games that have their Audio CD tracks ogg-compressed (e.g. Alone in the Dark 1 or HeXen II)
- or games that need some work to get them running in "pure" DOS but can in principle work (e.g. Lands of Lore II, Wing Commander 3 and 4)
- and _then_ some "remasters" or SCUMMVM packages like Z, Chaos Engine or Kyrandia-s which I would not recommend
I never bought anything off Zoom, but got Heretic and Hexen on Steam (before these were "kexed up") which run in DOS just fine.
3. This forum has exceptional curated lists of games that can be downloaded off internet (already linked above).
4. Finally, on the "second hand" - it does not need to be a single boxed game; there has been enormous amount of editions, reissues and compilations of PC games in last 30 years. To give an idea, in Europe I was able to get:
- Bitmap Brothers compilation with multiple old games on single CD
- Silmarils compilation by DotEmu - a single disk with a huge collection of games packaged to run in DOSBox that are perfectly runnable in DOS and Windows
- A Lucas Arts adventures collection and so on...
GA-G41M-Combo G41/ICH7 - Core 2 Quad Q9550 - DDR3 1033 - Radeon RX570 - CMI-8738 - X3MB - CM-300 - Saitek ST-30
Beetle/M/i815+ICH2 - Celeron 566Mhz - Opti 924 - YMF740 - MT-100
https://archive.org/details/peridot-26