Saint Bernard puppies are rarely available for free adoption. People or websites offering free Saint Bernard puppies should be thoroughly checked as they are likely a scam.
All pure-bred dogs come at a cost, and almost all rescue shelters require an adoption fee regardless of breed. A pure-bred Saint Bernard puppy can cost over $1,000; it is unlikely to expect to find anyone giving one away for free.
PetFinder.com provides a search engine to find adoptable puppies from shelters all around the United States. While shelters don't usually offer free puppies, the adoption fee is substantially less that the cost of buying from a breeder or pet shop.