If you have the core rulebooks for DnD, monsters shouldn't be a problem at all. The monster manual has a slew of them, along with its supplements. I'm sure you know that though, so...
http://www.dandwiki.com/wiki/Main_Page
This is the best place I know of for homebrew stuff like that. If you're just looking for cursed items that turn people into animals, you could easily make those up yourself. As the DM, your word is absolute; if a ring or pendant turns someone into an animal or monster, then it does, and you are the final say on that.