At the time of this writing, you will need a special insurance policy or be vaccinated against Covid-19 to meet the Costa Rica visa requirements, though this is subject to change. Here is the regularly-updated Health Certificate website where you can apply for the QR code that you will need at the airport.
Here is one such policy that does cover this, they will send you a visa letter once you purchase the plan. It only takes a few moments to apply.
There are also other carriers that may fit this requirement, such as
However, we don't have first-hand experience with these plans for satisfying visa requirements in Costa Rica in particular.
Once you purchase the plan, make sure you apply for the health certificate from the Costa Rica government. Don't wait until the last minute, or you could get into a tough situation at immigration.