Try
http://www.champdogs.co.uk click on Guided Search, then Terrier Group, then West Highland White Terrier. You will find a list of breeders listed with their areas, but please make sure any breders contacted do all the relevant health checks for the breed, particularly making sure their other dogs do not suffer from 'Westie itch' which can be due to hereditary causes.
The Kennel Club site also has a list of Westie clubs and the secretaries will be able to give you a list of breeders with puppies who are club members.
A good breeder will be able to help you with information on what you need to get before taking home your new puppy, and will be willing to give ongoing advice throughout the pup's life.