Puppy mill dog adoption

I am interested in adopting a senior/unable to breed anymore, puppy mill dog. I know that a lot of puppy mill dogs get dumped or worse, when they are too old/worn out to breed anymore. I know it will be extremely challenging. My husband and I are retired, and we both have had dogs since we were little. I didn’t think it would be this difficult to find one. I guess the unscrupulous puppy mill “people” aren’t exactly advertising. I even contacted the SPCA in PA and OH, as I know there are quite a few puppy mills in those states. There are also a lot of ethical breeders there, as well. Can anyone steer me in the right direction? Where are these dogs that the SPCA rescues? TIA for any assistance anyone can provide.