My ferrets are given medication for fleas by the vet but for years I
have made ferret hammocks and mattresses filled with dried lavender and
chamomile, mint and rosemary, roses and lavender.  Don't know about
repelling any fleas but it certainly makes for a sweet smelling fuzzy!
 
Any of the kitchen herbs that are pungent, such as catnip, sage, thyme,
garlic, and peppermint, would be beneficial in helping to repel fleas
if you want to try making your own to see if it helps.
 
Wildcat
[Posted in FML issue 4606]