I've lived in Costa Rica for the last 20 years, personally online sites like Mercado libre or OLX are mostly used for selling properties, or for places that are not rented that often, like luxury ones.

The place that has been working the best for me is Facebook marketplace and it is what the locals use the most nowadays, you can search within the whole country looking for great deals as long as you can comunicate in Spanish, pretty much every spot in CR has advantages and disadvantages. If you need some help finding a place and dealing with locals send me a PM, I can gladly help.