Gentle Giants Rescue has many large- and giant-breed dogs available as mixes, always as puppies. They are breeders, or purchase from breeders; there simply is no other explanation for the constant flow of dogs from this group, with absurd explanations like that they have them flown in from all around the country. They are bogus, through and through, charging over $1000 on the average per adoption- more than 4 times what any real giant-breed rescue would for equivalent dogs. They continue to maintain nonprofit status despite paying an absurd amount to their employee, and filing lawsuits to remove negative comments about their actions from the web.
Gentle Giants works hard to rescue large breed dogs. They have many nice and unusual dog breeds available for adoption. Do your homework on what breed would be best for you because the choices can be overwhelming.