Claro is the most popular telecom provider in Costa Rica. You can get a sim for $3 in any super markets or convenient store. The sim needs to be activated by calling a certain number, which only gives instructions in Spanish. Either you should understand Spanish or know someone who understand both Spanish and English to help you with the call and activation. People in local convenient store may not be familiar with English. We managed to find a kid who helped to call Claro and used Google Translator to communicate with the kid who then spoke to the Claro agent over phone. The entire process was quite a hassle. Our recommendation is to get it done either at the airport itself (with some extra bucks probably), or go to a proper super mercado which hopefully would have someone familiar in English
If you are travelling from US, T-mobile has 256KBps data speed in Costa Rica. But a local sim card is recommended because google search results with 256KBps is slow.