Nobody uses websites or agencies in CR, there is no need. This is all legal there so why set up with an unknown blind date based on pics that may or may not be real when you can just stop into one of the hotels or mp's and actually meet and talk to the girls and make your decision based on real info and chemistry. CR is not the puritanical USofA, this is not an underground secret society.
Although I have heard of a couple agencies that are legit in CR, so I am not calling it a scam, just saying it is unnecessary to take a chance of not getting what you are expecting. And agencies charge more.
Mako gives good advice. Costa Rica doesn't work like the USA. The hottest women will be found at the Del Rey in San Jose and the Cocal in Jaco. And there are a ton of other options. Take a look at costaricaticas.com for the most complete info. That said, if you do try that escort site, I hope you'll report back. The women look hot, but my guess is that the photos aren't representative of what you'd actually get.