GOG has most of them. A couple are even free. They used to have an official list of every title they carried that was compatible with ScummVM but the page appears to be gone now :/
Ooh! Beneath a Steel Sky came with my first cd rom kit (or at least I got it at the same time). It was gruesomely hard for me as I hadn't learned English properly yet. That's gonna be a fun walk down memory lane.