Apparently there are purebred rescue groups around the country that specialize in certain breeds, and there's a number from the website: 202-452-1100. It says to ask for the Companion Animals section and mention that you're looking for a breed-rescue group in your local area.
Additionally, according to the Humane Society, purebreds account for 25% - 30% of an animal shelter's population, which is a decent amount, so chances are pretty good of getting a purebred at a shelter.
There are also some websites for animal adoption shelters such as Pets 911, Petfinder, and 1-800- SAVE-A-PET. They all have Find a Shelter links, with listings of animals that are in the local shelters - there aren't as many as I had thought, which is a good sign, but maybe they don't have the time or ability to list all of their animals.
http://www.pets911.com
http://www.petfinder.org
http://www.1800SaveAPet.com
I have a blog about my journey to get a Boston Terrier as a pet;
you can read about it at http://bostonterrier.spot.blogspot.com.

