I've purchased one from the local petco store, the biggest size they
had at about $18.  Neither one of my ferrets seem to like it.  Rocky
will move it a bit, then curl up in the bottom and go to sleep.  Chewie
just sits and scratches at the inside of the ball trying to get out.
 
Get it from someplace with a good return policy if you buy one, just
in case they don't like it.
 
Steve
[Posted in FML issue 4521]