Doujinshi is amateur manga publications, featuring either a cast of original characters and plot or characters from another manga or an anime (a 'fan manga'). There's all sorts of doujinshi, though it isn't as widely circulated as more popular mangas, such as CLAMP works. Doujinshi is often cheap but is just as good as regular manga (well, some of the time...), and is sold on eBay, jpqueen.com, and other sites. Even if it isn't your bag, it's nice to take a peek and see a fan's perspective on Naruto... kinky pairings, too.
"I hunted for doujinshi all night..."