Hi Kim,
I can’t recommend any individual agents or real estate companies. But a quick Google search will yield several operating in the area.
Christmas to New Year’s is quite a time to visit Tamarindo. That week is when nearly all Costa Ricans head to the beach for family vacation, especially towns like Tamarindo, Jaco, Manuel Antonio. It’s going to be PACKED! I’d recommend visiting at another time if possible, especially if you want to check out real estate – nobody in the country is working that week for the most part.