Liver is available freeze-dried in most pet stores and it is a great training treat.
You can also buy it fresh in the grocery store to feed at home. Fresh liver can be cooked and then baked / dried to make your own liver treats. Liver is an excellent source of B vitamins (Thiamine, Riboflavin, Niacin, Pantothenic acid, and Folic acid), Vitamin A, and Vitamin K. It is also a great source of iron.
It is best to limit the amount of liver fed to your dog to not more than 1 g of fresh liver/Kg body weight per day.