how do they get names for dog ie jack russells?

They often name breeds after the place where they originated, ie Newfoundland, German Shepherd, Siberian Husky.

We shopped in some dog racing brochures once, and found lots of good ones.

Usually the breed club decides the name. Once a breed is established, meaning it produces consistent offspring, and a club of breeders is formed then they usually decide on a name and define the standard for conformation showings. The Jack Russell was named for an influential breeder. A few years ago the breed club changed the name and it is now officially the Parsons Terrier.