Paying for fucking is perfectly legal throughout the entire country. I don't know about agencies, but you'll find plenty of brothels and street walkers if you are going to a big city.

Ayyyyy, yo no se

I'm not sure of a specific agency to recommend.  I do know there are several seemingly upscale agencies, yet some only serve one part of Ecuador.  

I also know that Ecuador is full of amazingly gorgeous natural beauties.  I'm so jealous you're going!  I was born in South America and absolutely love the area you're going to  

Buena Suerte

xoxo
Lela

This is a very upscale agency:
http://www.firstclass.com.ec/index.php
Also, you will find upscale brothels in Quito, the capital such as a place called "Dollhouse"
Good luck in your search and be safe
Besos,

Paloma M.