We adopted a Shar-Pei through Shar-Pei Rescue, and we recently adopted a registered American Bulldog from another shelter. I frequently see pure breed Boston Terriers and other dogs in various local shelters, too. Sometimes these dogs have issues, which is why they end up in shelters, but sometimes the issue is that the owner didn’t understand what they were getting into with that particular breed, or with a dog in general.
We never found out much about our Shar-Pei, other than she was delighted to live with us, but we got the full background on the bulldog. She was a registered pure-bred someone bought because they are cool “guy dogs” and look tough, but he left her alone all day and she got injured. He neglected the injury so his father took her away from him and gave her to the shelter. We got her the medical care she needed but she will always limp. What a lot of folks don’t realize is that big tough-looking bulldogs are giant babies who want to sit on your feet all day.
As janbb says, check out petfinder.com but be aware that often the dogs shown online have already been adopted but the database hasn't been updated. Also check online for rescue groups in your area for the breeds you are interested in. Examples are http://www.savingshibas.com/ and http://adopt-a-corgi.adoptapet.com/