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In the US - I would say head to your local hunting/fishing/gun store and take a look at the fluorescent sights for bows/guns.

http://www.fiberopticproducts.com/Archery.htm

http://www.hivizsights.com/products/replacement-litepipes.html

Lots of different sizes/colors - cheap and readily available over here.

Plastruct sells it in red, blue, green and yellow - should be able to find that through local model shops (or ordered online from not so local hobby shops). Comes in 20" lengths and a half dozen different diameters.

After that - it is sort of hitting the various plastic suppliers till you find one that carries the diameter you want and the color you want. While I am sure there is someone who is doing it - I don't recall seeing anyone selling ones which are already cut to the right length for the Necron guns (though if they were - they would likely be tacking on a markup that makes it sort of ridiculous)