I cook my own liver treats for Bailey. simple and tasty. They are sort of dry but rubbery. no mess.
Just buy liver (preferably from a butcher, not supermarket, cheaper) boil it in water until cooked through. either whilst hot or once cooled, cut into cubes (size preference is up to you). some people put garlic in the water whilst boiling...I do this sometimes.
Store in sealed containing for up to 7 days in the fridge or can freeze and defrost as needed.
Really cheap way of doing it. I pay $3 for 3 livers, whereas to buy it $3.50 for a little bag (button bag). As long as you can handle the smell of offal cooking. Tip = open kitchen windows lol
Never met a dog yet who has turned down boiled liver. My girl will do ANYTHING for a cube
Hope that helps.