In reply to 00spaw: Lots of retailers listed here:
http://www.google.co.uk/products?q=platypus+bite+valve&btnG=Search+Prod...
It's relatively expensive simply because the tube & valve are the most expensive bits of the system. The Camelbak bite valves are pretty much the same price.
Whatever you do, don't get the new version of the platy bite valve with the on/off switch. It sounds like a good idea eg no need to worry about your platypus emptying itself in the boot of your car. In my experience it leaks at the switch, even on the off position.