The Disney Florist will have trees available beginning sometime in September. They are really cute and typically offer a couple of designs. The cost is around $375 though and if you want it shipped home afterwards, that's an additional $45. It's something you can keep and for that price, you probably would.
Aside from that, you can bring your own small tree and set it up in your room. If you do a search on Christmas Trees by thread date, you'll probably see a bunch of threads about a tree swap. DIS'ers will setup a list of those going during the holidays and one person will bring the tree, then pass it along to the next one in line. We've gone during the holidays and I picked up a 3 foot tree that had the lights and decorations already on it. I'm pretty sure I paid $20 at Target for it. You can ship it ahead of time if you want, pack it or purchase one from a local store when you arrive. There are lots of great options. Disney doesn't mind decorations, as long as you don't damage any of their surfaces. People even bring along suction cups to decorate windows and balcony doors.