The Ferret Store has harnesses.
http://www.ferretstore.com/harnessleads.html
These are the H-style harnesses with the snap closures, which is the only
type that I recommend.
 
The neck loop should be snug enough to barely slide over the head.  The
chest strap should be snug enough to allow only your index finger to fit
between the chest strap and ferret.  This is loose enough that a ferret
could eventually struggle free but snug enough to prevent quick escapes.
Never leave your harnessed ferret unattended or with his leash attached
to anything other than your hand.
 
Debi Christy
Ferrets First Foster Home
[Posted in FML issue 3176]