The best ferret tubes I have found come from the hardware store.  I have
two now, one for crawling and one for running.
 
The crawling tube is about 15 to 20 feet of 3" PVC pipe cut into different
lengths with elbows, Ts, and Ys and made into a maze.  My three boys,
Gilligan, Skipper, and Jean Paul, explore the maze until they become bored
with it.  When they do, I dismantle it and reconfigure it, and they regain
their interest.
 
The running tube is their favorite though.  It is a 10' firm plastic tube
that you attach to a downspout of a roof's gutter system.  As the boys
run through the dark tube, their nails clipping on the accordion pleating
creates a sound they love.
 
These hardware store ferret tubes last forever and are relatively
inexpensive.
[Posted in FML issue 3736]