Three people recommended Yolanda Santana, who works for Remax in Cotacachi. She was described as professional, efficient, and billingual as well as current, insightful, and on top of things. Her website can be found at http://www.remax.com.ec/ysantana.
Others recommended using Craigslist-Ecuador or simply googling Cotacachi rentals. Viviun.com was another website that was mentioned that sometimes has rentals in Cotacachi.