No visa needed to enter Panama if the following conditions are fulfilled:
As per EXECUTIVE DECREE # 521 of the Government of Panama:
Passport holders of any country that have one of the following: Permanent Residence Card, a valid multiple entry visa (that has been previously used), from the Canada, the United States of America, the Commonwealth of Australia, the Republic of Korea, the State of Japan, the United Kingdom of Great Britain and Northern Ireland, Republic of Singapore and any of the States that make up the European Union do not require a visa to enter Panama.
The visa granted by the countries mentioned in the previous article must be a multiple entry visa, has been used previously in the territory of the granting State and its validity shall not be less than six (6) months at the time of entering Panamanian territory.
If the above are not available visas can be obtained from the closest Embassy or Consulate of Panama. The letter of invitation can be generated by filling the form given:
here
More details can be found in Panama Government website : https://www.migracion.gob.pa/
