
This site in the paid for breeders section gets a good quality response. Also the KC's website -you get the option to have your pups listed on there when you register them for £20. Dogs Today magazine is very good as they only accept adverts for pups from health tested parents and demand copies of hip scores, eye tests etc as proof so the adverts are of an overall high quality, plus they are free.
Most of us though will make sure to have buyers waiting before we mate the bitch, but of course that can fall through -I had 12 buyers lined up and lost all but one when I had to change the stud dog at the last moment.