I travel on business twice a year to Havana and can assure you getting CuC's is not so difficult. If you are concerned about getting money for the taxi driver then rest assured, they are quite used to tourists having to go into their hotel and change up some cash before paying them. They do have ATM's but be warned, most UK bank cards are switched off for Cuba for fear of sanctions from the USA. My NatWest card has now been "switched on" but it took some time for my bank to do this. The sad news is that prices have rocketed in the last 3 years or so as even the capitalists in Cuba are taking advantage of the rush to go before the US really spoils it.