BakaBT is a private tracker that has lots of high quality anime/manga artbooks, soundtracks and extras like that. That’s all I use aside from Nyaa, but my needs are pretty basic in general

I just wanted to say that I love Rei.

Man, sexualizing Rei is so weird.

the specific artbook of the picture you posted is available on archive.org with the title “evangelion material”, you can download it separately with the torrent they provide. the internet archive actually has a pretty impressive artbook collection, though i actually found that specific file under a mirrored torrent on one of the dht search engines on the megathread, which are surprisingly helpful/extensive, though a lot of torrents (especially older ones) aren’t being seeded, but it’s a good start once you have the filename.

as for non-english forums, i’ve had luck with baidu search: you’ll be mostly looking for pan.baidu links in animanga forums/sites (which may be hidden behing login walls, but most of the time you can use google’s cache view), though you do need an account to download from them, which can be difficult to get depending on your location. one specific forum that seems to have a lot of content not available elsewhere is otomedream, but it’s 1) very difficult to get in, i’ve been on the waitlist to get my account approved for like two months; and 2) requires you to use their currency to get dl links to the actual content. also worth checking out is vk and japanese+chinese torrent aggregators and book download sites, fmhy has some in their non-english section. in most of these, you’ll have the most luck searching the original japanese title in quotation marks.

i can’t personally attest for private trackers like bbt and ab, but i imagine it’s worth checking out if you have an account.

I had found a bunch of complimentary materials to what I was watching on rutracker-dot-org. It’s not anime-oriented, but it can serve as a last place to search them.