Sarah this thread gives advice on what to look for in a reputable breeder -
http://www.cockersonline.co.uk/discuss/index.php?topic=48072.0 As said the only likely reasons why someone wouldn't KC reg puppies is if they are breeding from unregistered dogs (and therefore you have no way of knowing if these dogs are who the person says they are or of knowing their pedigree details or if any pedigree details you're given are genuine), if they are breeding from a bitch that is too young or too old or they've had too many litters from or they just don't want to pay to register the pups - no good breeder would do any of these things.
If you're looking for a puppy try contacting your local representative of the Cocker Spaniel Club, they should be able to give you the details of some good breeders in your area -
http://www.thecockerspanielclub.co.uk/breedclubs.htm Alternatively you could go along to a show in your area - details of many upcoming shows are posted on the Showing board on here - and speak to some of the exhibitors there. Free ads, classified puppy selling websites etc. are not a good way to find a puppy, I don't know of any reputable breeders who would sell puppies in this way so steer clear of those.