Check out kemono.su / kemono.party. They're a patreon(among other subscription service) archive site that makes use of crowdsourced keys. It'll initially look like 110% porn, but you can often find other patreons there if you search those specifically, such as those for 3d printing stls. It can sometimes be hit or miss whether they properly archive downloads, but they're usually a worthwhile option.

I don't have a specific place; just some of the private general-purpose trackers I'm on will occasionally have someone come by and dump a pile of STLs from various places on them.

(The private part, unfortunately, means I can't actually share more, sorry - fight club rules and whatnot.)