You could try www.apuntavamos.com, I'm just not sure they have an English version of it. FYI "alquiller" means rental.
For New Year's the rentals are in "fin de año" periord. its nowhere in the site, but that's from the 26th (sometimes the 27th) to the 1st of January (which is bad this year, since it's a saturday). You can try to negociate staying until Sunday or any other day, but they usually charge for that (and they also consider it another day if you don't leave until 11am).
I was told people were suppose to leave on the 1st because that's the day people who rent for the entire month of january (or half of it) arrive.
Anyways, I hope that's of anyhelp!
Just be prepared to spend a lot of money because everything is quite expensive at this time of the year!