We had a storm last month and a lot of trees were pushed over from the wind. I keep a saw on the back floor of my truck.
here's what I look for.,,,(see pic below).. I'm always on the lookout for uprooted trees if you get lucky there will be a perfect one...or you can hot glue some branches on a good "trunk"....do you have any wooded areas near you? there has to be some fallen trees. investigate where the local public works takes them? you just need a little luck, just be careful you use hardwood and not pine. a lot of times the roots have no bark so all you need to do is boil them a couple hours and carefully scrub. but I find the pressure washer with NO SOAP works best after boiling, I wouldn't mind shipping some .. if I get out and find some good ones I'll let you know.. They don't have to be Oak, just not pine. go out and take a walk and you just may find what you need. I think you can use branches without any problem. try and find dead ones. peel back the back to expose the wood... if you can easily push your thumbnail into it throw it back, needs to be hard. I like the roots because they seem to sink right away. but I still glue them to a small flat stone to help with placement. the one tree I did is 6 pieces glued together... let me know if you find anything...and I'll be looking next time out. how tall and wide do you want your trees?