The only one I know of is www.practicalpdf.com run by Joel Geraci. Joel also published quite a lot on the technical details of creating a Navigator on his blog when he was an Adboe employee ( pdfdeveloperjunkie blog).
Adobe did thier best to get programmers excited about custom Navigators but I don't think they had many takers. And as you have found out there is a specific skill set required to develop them, so anyone who is doing this will most likely offer them for sale, not free.
Hope this helps,