Part one: Source sites.
These are sites with actual original content. If you want to get down and dirty you can search them directly, especially if you're fluent in Japanese, but for just general browsing you should probably go for one of the posting sites listed below, unless you have something very specific in mind.
1. Pixiv.
http://www.pixiv.com/The majority of my porn almost inevitably comes from Pixiv, since it has an excellent artist culture (Unlike deviantART) and is very, very popular. It also has a lot of things that aren't porn that are still pretty cool (You should see my bookmarks...). Get an account, it's a must-have.
2. Nico Nico Seiga.
http://seiga.nicovideo.jp/This isn't mentioned much, but it's a treasure trove for all sorts of interesting images. Again, get an account.
3. Nijie.
http://nijie.info/Yeah, there's a lot of dick jokes and arbitrary crap, but if you know what you're doing you can find what you want fairly easily. I generally don't have much from Nijie, actually. Again, get an account if you plan to go here.
4. deviantART.
http://www.deviantart.com/The only reason why I'm posting this is because some of my ecchi is from there. Otherwise it is completely awful and should be avoided unless you know exactly what artists to keep track of.
5. Tinami.
http://www.tinami.com/I guess this doesn't quite count since Tinami isn't a porn site. That said, it's got some things that are probably at least somewhat /e/-relevant, I think.