A good source for rigid plastic tubing is United States Plastics,
tel. 1-800-537-9724.  Their web site is http://www.usplastics.com/

I have their catalog and they sell rods, tubing, sheets, pipes,
bottles, etc.  They also stock all kinds of liquid pumps, valves and
connectors.  The catalogue is 250 pages and is packed with all types
of neat stuff.

Regards,
Roger Hughes