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.
This is the answer. There are free indexes out there like https://binsearch.info but thanks to automated DMCA takedown bots, there’s very little on “open” usenet that stays up for long. You need to get on a private index and, unfortunately, pay for a subscription to get any kind of reliability.
I use audiobookbay. I don’t trust them enough to “sign up”, so I just snag the hash from the description and edit it into a magnet link.