Lonely Planet seems to indicate that it is possible to get a visa on arrival at Freetown. However, I have not been able to find any other references to visa on arrival at an airport in Sierra Leone. Is this a possibility for someone on a US passport? What are the fees and requirements?
I traveled to Sierra Leone about half a dozen times in 2010 and 2011 and got my visa upon arrival every time, without a hassle.
I remember having to show my yellow fever vaccination booklet once or twice. Besides that, I don't remember any other requirements. Cost was in the order of 50 USD, but my memory of that is vague.
That said, immigration rules do change, and this was three years ago.
As of 21 February 2017, Timatic, the database used by airlines, states:
Passengers with a confirmation from immigration that a visa has been approved before departure can obtain a visa on arrival.
In other words, you need to pre-arrange a visa on arrival and receive a confirmation letter.
While it is possible to get a VISA upon arrival, the main issue may be whether your airline allows you to board a flight for Sierra Leone without a valid VISA. Check with your airline to determine whether they require one. I've flown with AirFrance/Delta/KLM and they require a visa as you check in.