You probably won't see celebrity textures and characters for sale, due to copyright/trademark concerns. But there are lots of free ones. Star Trek characters, props, and sets seem to be especially popular. Often, when people do celebrity characters, they disguise them slightly. For example, a Scully character was posted to the free section, under the name "Federal Agent." There's a character called "Arrogon" for sale, who bears a certain resemblance to a certain LOTR character. I know there are Poser artists who do Harry Potter art. Your best bet for finding them would probably be Harry Potter web sites. IME, media fans tend to post their stuff to their fan sites, rather than to generic forums like Renderosity. I'm not a HP fan, so I can't point you to specific sites, but I know some fans were using Poser to create illustrations for their favorite HP fanfic.