Mark,
I had the same problem you had when trying to find Sustical.  I tried all of
the pet stores.  Then someone told me that it can be found in Drug Stores and
grocery stores.  It's a non-lactose based dietary supplement for humans.  It
comes in a lot of flavors.  I use vanilla, but I hear that some ferrets like
strawberry.  I guess any flavor except chocolate will do.
Dick B.
[Posted in FML issue 0989]