Bully sticks are great. Costco sells packs of them for the best price I've found.
Pig ears are good too but don't last that long. Moo tubes (cow trachea) they love.
Some of mine also like the femur bones. Those last a long time, and they even chew them after all the tasty stuff is chewed off.
Check out Pet Deli. They have great chews. http://store.petdelistore.com
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