Chicken wings are cheap, (but usually cheaper at supermarkets) so are pork ribs. Butchers sometimes have chicken necks too. Always worth asking for bones, some butchers give these away free. If you just want meat, no bone, just say it is for the dog and ask what's cheap that day. All types are suitable. They usually have various types of heart, not always available in supermarkets.
I get my offal, chicken carcasses and necks, goat mince, rabbit, pig/lamb/ox/chicken hearts and bones from the butcher, plus anything that is reduced. Pork and lamb ribs, chicken portions, oily fish (usually sardines) and frozen mince comes from Sainsburys. Tripe comes from the pet shop.