Hollow threaded rodsizes

http://www.ebay.com.au/itm/2x-M10-40mm-x-10mm-Allthread-Hollow-Threaded-Rod-Tube-For-Electrical-Lamp-Socket-/361582816343 .

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hollow threaded rod3/8

I'm having a hell of a time trying to find custom sizes other than that elsewhere if anyone has a website that I can find the bolt in the size I need. The bolt is to hold a light on a motorcycle. The wiring goes through the rod.