i get fish heads for free from the fish man at the market, dogs love them, ferrets werent impressed with them though, they prefer rabbit or fresh killed rooster.
also, i take away unwanted roosters for culling for free, few minutes later its skinned and gutted, dogs love it! freshest chicken there is!
whole rabbits, whole chickens, fish heads and bones from the butcher.... all free, if your in a bad way financially and are shrewd about it theres no reason you cant keep dogs for next to nothing, couple of cheap bread loafs from tesco and job done
also feral goat is a good addition to