Mountain Dew is available in Costa Rica, but it's not as ubiquitous as in the U.S. Here are your best options:
Larger chains like Auto Mercado occasionally stock it, usually in the imported beverages section. Availability is inconsistent, and prices are significantly higher than in the U.S. (often $2–$3+ per can).
Several e-commerce platforms ship to Costa Rica:
Most expensive option due to shipping/import fees, but most reliable.
Travelers from the U.S. can bring up to 1 liter of alcohol/beverages duty-free. Pack cans in checked luggage (pressurized cabin safe). Cheapest but least convenient for residents.
Costa Rica has excellent local sodas: Frescolita (cream soda), Ginger Beer, and tropical fruit sodas (guanábana, maracuyá) that many visitors prefer.