Visitors to Sierra Leone must obtain a visa from one of the Sierra Leonean diplomatic missions unless they come from one of the visa exempt countries.
Visa exemption
Citizens of the following 15 countries can visit Sierra Leone without a visa:
A maximum stay of 1 month without a visa is allowed for holders of diplomatic and service passports issued to nationals of Iran.
An agreement between Sierra Leone and Saint Kitts and Nevis on mutual visa-free visits for 90 days was signed in February 2017 and is yet to be ratified.
References
Visa policy of Sierra Leone Wikipedia(Text) CC BY-SA