
Hi,
The price range starts around £600 for a good quality puppy but it can vary.
The way in which we found our breeder (Different Breed) was to join the Breed club, the link is here
http://www.the-yorkshire-terrier-club.co.uk/.
They often have links to breeders on their site and your newphew's wife could try to get out to some shows and have a look and talk to some breeders there :)
Also check out the champ dogs breeders page as there are lots of websites and contact details to see if there are any litters coming up.
Good Luck