The most easily exchangeable currencies in Sierra Leone are US dollars, UK pounds and euros, in that order. Large denominations get the best rates. Forex bureaus (and street traders, though avoid them unless somebody you trust makes the introduction) invariably offer better rates than banks.
You can rarely pay with a credit card in Sierra Leone, but some Rokel Commercial Bank branches give cash advances (up to US$2000) on Visa cards, and ProCredit Bank has ATMs in Freetown that spit out up to US$100 per day for those with Visa credit and debit cards. Don't rely on them too heavily, however, as they sometimes don't work.