Earlier quoted context omitted.
Some people on Reddit are recommending Pixiv, a Japanese site.
Pixiv is also a cool company. I don't want to speak too much for fear of messing up some of their story but they're basically a bit like Japanese deviantArt. They've open sourced some small parts of their code: https://github.com/pixiv As far as content goes, it is constrained by Japanese law, which is mostly more liberal around art with the exception of their oddly specific yet vague censorship requirements.
Japanese culture has a very strange aspect to it, though. It is a rule following culture and they don't readily change the rules. Once you have a law for something, people don't question it. It's the law. It doesn't matter if it's a good law or a bad law. It's just the law and you make do. None of those laws changed in 70 years and unless some outside force puts pressure on the government, I suspect they won't change for another 70 years.
I always find it amusing that Americans, especially, complain about protectionist agriculture policies of Japan. Or American pundits show surprise at the really bizarre censorship laws. All of it was put in place by the Americans -- albeit probably without realising that once a law is in place, it's gong to be there for a long time :-) This is American culture fast forwarded in time 70 years!
WRT to the liberal laws regarding art... well, generally about what constitutes child pornography: if you are drawing something you have pretty much free reign to draw whatever you like. This is (IMHO humorously) constrained by the specific censorship laws (I suppose the 1940's American occupying forces did not consider child pornography). Artists will follow the letter of the law, but then will feel free to draw whatever they want outside of those boundaries.
I believe this is due to a very strong feeling in Japan that what you think is private and up to you. Even discrimination is famously not illegal in Japan. For example, you are free to serve or not to serve any customer for any reason in Japan. It's a pretty big deal for most people. Remember that before the war Japan had no censorship laws. You could depict anything you wanted in art. The current state of affairs is just an extension of that, tiptoeing around the censorship laws imposed by the occupying forces in the 1940's.
Child pornography is reviled here in Japan to pretty much the same extent that it is anywhere I have ever lived (as well as art that depicts rape, or other violent sexual crimes). However, if you draw it rather than photograph it, it is not illegal. If it doesn't involve anyone other than the artist and the viewer, it's nobody's business but theirs. Talking about it publicly is completely not acceptable in normal society, but thinking about it is your own business (even though most people will privately judge if they think you are the kind of person who is interested in that stuff -- but again, that thinking is private and not expressed publicly).
I'm always hesitant to talk about this stuff on a public forum because I don't want to incite a large thread about what should or should not be classed as child pornography. It's one of those things where no matter what opinion you hold, you are going to be seen as a monster by people who hold a different opinion. I just thought people would be interested in some information about why the rules are so strange in Japan.