Yes I agree with Pixie on the vet idea, there are alway ads in our vet's surgery.
We have always got our dogs from a local rescue service, and it has cost us nothing as opposed to the RSPCA's £30,[last I heard].
If you hanker after a specific breed of dog then all of these breeds run their own rescue service rehoming dogs of that breed. Look them up on the net.
At least that way you would find out why the dog is homeless, and not just be lumbered with a dog with unknown problems. In many cases there are good reasons why a dog needs a new home, but beware of the 'problem dog'. For many years of my life I was involved with breeding and showing dogs, and I have seen some sad cases, so caveat emptor Lynne.
Giok