I disagree with a bitch needing to have one litter before being spayed.

That being said here's a link to the American Shetland Sheepdog Association:
http://www.assa.org/
There is a breeder referral list on there to get you started on looking for a breeder (but I agree with everyone that stated that a responsible breeder won't sell a pet pup without a contract saying it's not to be bred).

Also on that site:
http://www.assa.org/shouldI.htm