Cut up some Ox Liver into VERY SMALL pieces (you might find it easier to freeze the liver, partially defrost it, then cut it up), place on a plate (keep separated) and microwave until the pieces of liver are hard.
You can put the cooked liver pieces into a bag or your pocket to use when required. The dogs absolutely love them!
These will keep well in an airtight container.
If you want to buy some liver treats look at the following site -
http://www.dogtraininginfo.co.ukI recommend the refill bags of liver treats and garlic and liver treats.