On a recent journey to Sierra Leone we utilised the visa facilitation service provided by Visit Sierra Leone (VSL Travel) in Freetown. We assume it would be cheaper to apply for a visa at a Sierra Leone embassy if you have access to one, but this option was not available to us.
VSL Travel have an arrangement with the immigration office so they can take all the hard work out of the process for you and within 3 days we had a visa that enabled us to enter Sierra Leone with zero hassle. We crossed overland at the Guinea-Sierra Leone border crossing at Pamalap/Kambia.
Here is a link to the VSL Travel website and this page provides you with a detailed list of the process and costs for various countries. VSL Travel only assist in facilitating the visa process and the final say on whether you receive a visa or not remains in the hands of the immigration office.
We have had confirmation from several people that the process with VSL Travel was very simple and ultimately successful.