depends on what you want but I would always check your local animal rescue shelter (Cats Protection League, RSPCA, Blue Cross, PDSA etc). They often have kittens in. The animal are fully innoculated for their age and are neutered (if they are old enough). Some are even micro-chipped.
TBH, I wouldn't buy from a breeder unless you were certain that you wanted a particular pure-breed and were sure that the breeder bred his/her animals in a humane and acceptable way (be sure you see the parent cat and where the animals live etc). Word of mouth recommendations go along way when looking for a breeder