Breed-specific rescues, such as Cairn terrier rescues, are usually private organizations and can be found through local Cairn terrier breed clubs or online searches for "Cairn terrier rescue" and your location. The American Kennel Club also has a list of breed rescue resources.
As of 2015, the AKC's Cairn terrier rescue page directs to the Cairn Terrier Club of America, which has a rescue branch. There are also terrier-specific rescue clubs that do not focus on Cairns but may have them in often. AKC-registered Cairn terrier breeders are another good source for finding local Cairn rescues, as most reputable breeders have some connection to breed-specific rescues.
If you are looking for a Cairn terrier to adopt, you can also look at local shelters that are not breed-specific. Cairn terriers are a popular type of terrier and often end up in municipal shelters or other all-breed rescues. You can use sites like Petfinder.com to search by breed or contact the rescues directly.
If you are looking to volunteer with Cairn terrier rescue, contacting your local Cairn breed club is the easiest way to find any Cairn terrier shelters in your area. There are not Cairn rescues in every area, but the breed club can also help you find Cairn rescues interested in expanding into your area. Even if there are not regular volunteer opportunities, rescues often need transportation assistance and emergency fosters.