You can also look at Vrbo.
This has retnals-- but I think more long term options: https://www.point2homes.com/
As for Facebook Groups, I would look for specific smaller groups in regions, like these: https://www.facebook.com/groups/puertoviejovacationrentals
https://www.facebook.com/groups/756500642604738
https://www.facebook.com/groups/1990512857886393
To be honest, for short notice, a few days short term rentals, I would use Airbnb and Booking.
Are you heading over to the Tamarindo area? Feel free to send me a message on WhatsApp if you are +506 8962 9593 and we can meet up for a sunset. 🙂