The Disney Florist does offer a decorated tree but the prices are quite high -- $150-$200 if memory serves. Many people have brought smaller trees with them or bought them in the Orlando area (although that might be risky if you are close to Christmas on arrival). An option, is to go with the smaller tree and also do some Christmas floral arrangement from the Disney Florist. They usually offer several options. Many people also string lights either on their balconies and/or around the windows.
I did see one family who brought a U-haul type trailer down to OKW that had their full size artificial tree and other Christmas paraphenalia....including presents I assume. Of course, that's only possible if you drive.
Christmas at WDW is wonderful -- enjoy!