In real estate the saying is "location, location, location." Regarding getting a Norwich terrier the saying is "timing, timing, timing". Norwich have small litters. (The average litter size is two to three.) So it is not unusual to contact a breeder when they do not have any puppies or older dogs to sell. But if you are persistant and patient, you can get a Norwich terrier.
The best way to find a Norwich terrier is to individually ask breeders if they have any Norwich for sale. Try to be as specific as you can about what you are looking for - age, sex, color, etc. (See Some things to consider about Norwich terriers.)
I suggest you start with breeders who are members of the Norwich Terrier Club of America (NTCA). The NTCA web site has a "Directory of NTCA Breeders".
Not all Norwich breeders have web pages, but many do. I maintain a list of web pages of Norwich breeders. Warning - not all breeders are reputable breeders. Buyer beware! Do your homework!
The American Kennel Club has an online Marketplace where breeders can list puppies that are available. Again - buyer beware, and do your homework.
I strongly urge you to avoid generic web sites that sell puppies. You are likely to get a Norwich that is not AKC registered ... or worse is not even a purebred Norwich terrier.
Last updated 20170330