Which forum would I to go to find out info on tree houses
Look thru all the titles and see if someone looks likely. I don't think there's a specific one or that it would necessarily have a big following.
David, are you asking about one that you can live in or for kids? I have been fascinated with them. Don't know of a forum for this on GW. But a link below is from a search. There are tons of ideas and blogs from those who have built one.
Here is a link that might be useful: Building a tree house