I gave up on finding a tracker for STLs. Almost none and no available invites. I have, however, found a wealth of them on Telegram and it's not hard to find. There will be lots of ads for crypto subscription channels. There is maybe one that's actually any good and/or not a scam. There is a boatload of scams tho. There are also channels that focus on group buys. Do your research but those are also important because there are many artists that need support to keep up their work.

Also watch out for sites selling STLs for stupidly cheap with vague or non existent attribution. None of your money will go to the artist

I didnt even know there was trackers for 3d printing. Does materialize have any public signups / interviews?

I've never seen an invite pop up for materialize anywhere. I'd gladly trade for one, but I guess it's fucking unobtanium. I've not found any other sources either, and I've looked.