First contact person for questions about trimming is the breeder of your Westie, most of the breeders offer trimming for the dogs from their kennel. This way they keep contact with their offspring and owners.
But every Westie owner is of course free to find someone to trim the dog professionally. We normally do not recommend visiting any dog salon. Dog groomers are all-rounders and are able to groom any dog, but they are often not experts in the treatment of harsh terrier coat. And unfortunately, bad trimming can do more harm than good to the dog, for example if the hair is not plucked out but capped, or if the hair is only trimmed superficially and then clipped. Not to mention the fact that the typical Westie hairstyle with the round head, the pointed tail and the right length of coat on legs and body simply needs experience and practice.
List to follow!