Find someone with one in their backyard, ask them if you can shoot it. Wait for their expression, so you know if you need to offer them money or not. You can even make something up about using it for your college class or film school etc.

Yes...find one in someone's tree then shoot a couple of exteriors of it. Then build the set for the interior somewhere else...thay way all you need is to get those exteriors. If not then you will have to build something that has two sides and a bottom so you can then cut to the interiors. Once you've established it is a tree house then you don't need those exteriors anymore unless you have shots requiring it. Shoot around your problems. I think "Stand By Me" had some treehouse shots. I don't recall how much of the exterior was used.

True; if you can get away with only shooting a handful of exterior shots then convincing someone to let you shoot them should be much easier than actually having to film inside a real tree-house. I'd imagine that shooting inside it would be difficult, to say the least.

If you know of a dealer who has one set up as an example of what you're buying, it's quite possible that you could talk them into letting you shoot those shots in return for a credit in the movie; I've found a lot of companies are happy to help out so long as you ask nicely and won't cause much disruption. Just make sure you finish the movie and give them a copy!