Cocker spaniels are very popular dogs and you will often see them advertised in local papers. If several breeds are advertised at the same time it is possible that the advertisers have just bought in the litters from puppy farmers. The best way to find a puppy is by recommendation. There are several cocker spaniel breed clubs throughout the country and the secretaries of these will be more than happy to give you some contact numbers.
You can find a list of all the regional breed clubs on the Cocker Spaniel Club website HERE