Most of mine were bought when they first came out but after that, GameTZ was my goldmine. You can trade or buy. Not sure if they still have free accounts but they not only have PC games but other consoles as well along with movies and music cd's.
Eco Quest is EXCEEDINGLY rare. So much so that nobody's actually seen a copy. It may not exist. But that's strange considering it has a SCI10 release, of which all games running that particular version of the interpreter have a 320x200 16-color counterpart. But it did also get a second floppy release in the SCI11 interpreter which has a built-in 16-colour 640x400 driver. Perhaps it came out relatively quickly and so didn't bother making an actual separate EGA version.
I had copies of KQ5 and SQ4 in EGA because my dad had a Tandy 1000 at the time and had to send away for the EGA copies. Also had Rise of the Dragon and Heart of China. I acquired the others eventually as well. But they are indeed rare.