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Where can you buy extra lengths of it from?
Got to a place that sells air tools and equipment. They use PTFE tubing, and you can get it in small diameters. (I use it as a inner cable friction lining). You may not get red, but you'll get something suitable.
If you want to give uncoloured tubing a colour, it should be possible to dye it with Dylon.
Edit:
Just found this [url= http://www.sjscycles.co.uk/transfil-cable-waterproof-kit-prod24401/ ]from SJS Cycles[/url]
Perfect thanks! black will be better than red!
[url= http://www.ebay.com/itm/like/190972500129?lpid=82 ]Jagwire seal liner[/url]
I use mine with Shimano endcaps, minus the rubber boots.