I have never seen a real Christmas tree. We have a fake one, the bazaars sell lots of decorations. The nicest store for Christmas, in my opinion, is Alparamis -- the one in Zona Norte is better than the one in Galerias Pacifico. At the one in Zona Norte they have visits with Santa. The entire place is decorated with Christmas trees and they have fir boughs (fake) hanging from all over the ceiling. The ornaments they sell are nice, if pricey.
There is a holiday village on Libertador now. From the looks of it, this year won't be nearly as hot as usual, so it may be easier to get into the Christmas Spirit!