First of all, you will need to know that you will not find Cuban Convertible Pesos in any country except Cuba. This currency is not internationally recognized, not used. So you will obtain and use it ONLY in Cuba.
There are two official institutions where you could exchange foreing currency into Cuban convertible pesos: Banks and Casas de Cambio (CADECA). We strongly recommend that you do not exchange currency on the streets.
There are some scamers that use old (pre-revolution) bills as Convertible Pesos and try to exchange them for your currency, usually offering better exchange rates. We also know of the existence of fake CUC bills, and coins. It is very common the existence of fake $1 CUC coins, so if you will receive change, after buying something, alway ask to receive $1 CUC bills instead of coins. It is very advisable to always have small ($1, $3, and $5) bills, instead of big ones ($20, $50, $100).
You could exchange some currency in the airport at your arrival to Cuba. You will find that the rate is slighty less favorable to the exchange there, perhaps to take advantage of the fresh tourists in need of convertible pesos. So exchange there just what you will need the first day. You could exchange the rest in any institution, banks, or CADECA all around the country, the rates will be almost the same anywhere.