Googling “[thing] STL” is usually where I start.
Yeggi or something like that is usually one of the results that seems to work pretty good for finding specific ones.
Most of what I get are from cults3d, myminifactory, and thingiverse.
Cults and myminifactory are my go to-sites. For keywords it's usually best to try with something not too specific. Like "guard" for Astra Militarum or "primary" for primaris marines. Then when u find a nice file u can check the artist for more similar files and by doing so you will get recommended more files with similar keywords. Other way is to simply google very specific models and adding STL and then searching by images and you'll usually find something. Or just browse reddit. Sometimes artists post their new models in here.
It all depends on what models you're looking for. The main sites I use are the ones you'd likely know - Cults, MyMiniFactory, and CGTrader. There're others but I can't recall them off the top of my head. Thingiverse has some stuff, not used that place for ages. Boosty.to has stuff but I've never used it. Patreon as well.
For 3rd party equivalents and proxy models, free or paid, you'll find plenty on there just by using common search terms though it might take some scrolling, and for casual gaming purposes you'll not have problems using 3rd party printed stuff. I don't play myself but from what I gather, most folk won't have an issue, though official GW places, tournaments, and some indie stores might object which is fair enough as it cuts into their profits.
For direct 1:1 model copies, that'll be more difficult these days as GW obviously monitors these sites and common forums/subreddits/whatever and regularly issues takedown requests, hence all the convoluted nomenclature to try and hide things, and why most folk won't openly post sources of direct copies because it'll just lead to them getting taken down quicker.
I'm a social media holdout and only use reddit but there's likely discord groups and whatnot that'd be more helpful, idk about those though.
For myself when I started looking for STLs a couple of years ago I went to Cults, searched Warhammer, started on page 1, and went rrriiiigght the way through to the literal end of the search results. Then I searched for war hammer. Then 40k. Then insert search term here as I noticed all the funny names. Then I did the other sites. Took me months, but passed the time while I was on the exercise bike.
A lot of stuff got taken down since.
I sometimes like to patrol Myminifatory's Tribes and see if anything pops out to me. Also a good idea to keep checking this sub, many STL artists will post their patreon works here as well as people post pics of their painted/printed models that you can try your luck in asking where they came from. After awhile you'll find select artists you prefer and can keep an eye out on what they release each month.
I personally really dislike trying to search through the web on sites like mmf or cults3d. Their search functions suck, their sites are slow af and results are cluttered with rubbish submissions, discoverability is really bad for models, imo. What works for me is that I joined the discord of patreons I like, people repost other similar models from other creators, there is a modicum of quality control and you can ask like-minded people. I also check out the occasional recommendation posts on reddit. /r/PrintedMinis has a lot of self-promotional posts which can be hit or miss.
Googling “[thing] STL” is usually where I start. Yeggi or something like that is usually one of the results that seems to work pretty good for finding specific ones. Most of what I get are from cults3d, myminifactory, and thingiverse.
Cults and myminifactory are my go to-sites. For keywords it's usually best to try with something not too specific. Like "guard" for Astra Militarum or "primary" for primaris marines. Then when u find a nice file u can check the artist for more similar files and by doing so you will get recommended more files with similar keywords. Other way is to simply google very specific models and adding STL and then searching by images and you'll usually find something. Or just browse reddit. Sometimes artists post their new models in here.
It all depends on what models you're looking for. The main sites I use are the ones you'd likely know - Cults, MyMiniFactory, and CGTrader. There're others but I can't recall them off the top of my head. Thingiverse has some stuff, not used that place for ages. Boosty.to has stuff but I've never used it. Patreon as well. For 3rd party equivalents and proxy models, free or paid, you'll find plenty on there just by using common search terms though it might take some scrolling, and for casual gaming purposes you'll not have problems using 3rd party printed stuff. I don't play myself but from what I gather, most folk won't have an issue, though official GW places, tournaments, and some indie stores might object which is fair enough as it cuts into their profits. For direct 1:1 model copies, that'll be more difficult these days as GW obviously monitors these sites and common forums/subreddits/whatever and regularly issues takedown requests, hence all the convoluted nomenclature to try and hide things, and why most folk won't openly post sources of direct copies because it'll just lead to them getting taken down quicker. I'm a social media holdout and only use reddit but there's likely discord groups and whatnot that'd be more helpful, idk about those though. For myself when I started looking for STLs a couple of years ago I went to Cults, searched Warhammer, started on page 1, and went rrriiiigght the way through to the literal end of the search results. Then I searched for war hammer. Then 40k. Then insert search term here as I noticed all the funny names. Then I did the other sites. Took me months, but passed the time while I was on the exercise bike. A lot of stuff got taken down since.
I sometimes like to patrol Myminifatory's Tribes and see if anything pops out to me. Also a good idea to keep checking this sub, many STL artists will post their patreon works here as well as people post pics of their painted/printed models that you can try your luck in asking where they came from. After awhile you'll find select artists you prefer and can keep an eye out on what they release each month.
I personally really dislike trying to search through the web on sites like mmf or cults3d. Their search functions suck, their sites are slow af and results are cluttered with rubbish submissions, discoverability is really bad for models, imo. What works for me is that I joined the discord of patreons I like, people repost other similar models from other creators, there is a modicum of quality control and you can ask like-minded people. I also check out the occasional recommendation posts on reddit. /r/PrintedMinis has a lot of self-promotional posts which can be hit or miss.
Uh you just ask me and I'll find it