There are some dog treats that may be available in your area pet stores:
dried liver, heart, lungs and kidneys (sold in separate packages).  Those,
manufactured by a Canadian company, are not flavored.  You can also find
dried garlic flavored liver.  I purchased the dried heart: I think it would
be better than the liver which would be rich in vitamin A (I give my ferret
diluted Ferretone) and because drugs might be accumulated in the liver.
 
Those treats are great because: they are very rich in proteins, it is the
dried organs so they are not processed, contrary to most dogs or cats treats
that are made of mostly wheat or corn with some meat.  It will not spoil if
your ferret hides it.  However, your ferret might still prefer raisins and
you may have to get him used to those treats.
 
Lorraine Tremblay & Pepe La Pew, Mukluk, Sanka, Presto, Snoreau & Boum Boum
[Posted in FML issue 1734]