there are so many to choose from but often they have little selection or are poorly managed. i found a nice one:
http://www.holiday-velvet.com/buenos-air...
Try here.
http://www.travel-living.com/ingles/v_in...
Good luck with your move, and enjoy the city!
Try Craig's List:
http://buenosaires.en.craigslist.org/vac...
or Zonder:
http://www.zonder.com/vacation-rentals/A...