About 20 years ago I moved into a house that had LOTS of cockroaches.  I
bought a $7 can of 'Pruff' at the hardware store.  It is boric acid and
it said safe for plants, animals and kids.  It is a blue powder that you
put a teaspoon inside drawers, cracks in wood, anyplace a creepy critter
would crawl through or hide.  It takes up to three months to work because
new eggs are hatching, but after that I had NEVER seen another cockroach
in that house and I lived there another 5 years.  I didn't have ferrets
then, so you will need to reed the ingredients.  It has been a long time
and they could have change it.  If in doubt, run the list of ingredients
past your vet.
[Posted in FML issue 4684]