Not sure exactly, although Tamarindo does have quite a few surf shops that carry alot of stuff. They even have a Robert August shop that sells his epoxy boards. When I looked at buying a board in Samara it was $380USD, used 7'6" fibreglass, not horrible shape but some serious pressure dings on the top. Way too expensive in my opinion. Since then I just pay the cost and take my boards that I am used to with me. It actually ends up being cheaper than renting for me.
Here are some names of shops in Tamarindo, see if they have websites or phone numbers. Also check out
www.tamarindo.com
-Iguana Surf
-Tamarindo Adventures
-Blue Trailz
-Robert August Shop (in the Tamarindo Vista Villas hotel - 1800-292-3786
www.tamarindovistavillas.com owner is "SurfnMoose", from Santa Cruz, Cali, supposedly will help with any questions you have)
-Matos Films Surf Shop - supposed to have lots of used boards
If you still can't find what you need, try Chad Fisher @ 1st Choice Realty
506-653-1132
www.playagrandecostarica.com
Hope some of this helps.