I found a few looking around on custom computer cabling makers/sites promoting to the pc building market. Only issue is that most of them won't go smaller than 6 inches (might make smaller on request but that will probably run the cost up a fair bit.)
This is the only one I could see already made that is about the right size: http://penonaudio.com/OTG-Pure-Silver-Cable
also http://www.pexonpcs.co.uk and http://www.keyclack.com both have cables you can get made, but they only go as low as 6 inches. Keyclack also sells a DIY parts pack to make your own with decent components (as opposed to cutting up cables you have already).
Personally, I'm going to try making one, as it'll be less irritating when I need to replace or fix them. It seems reasonably straightforward, just stripping and some finicky soldering really and I have a few decent longer usb cables lying around that I can cannibalise for parts. Going to use some flexible putty for the connectors, which should fix the eventually breaking of the rigid connectors you get with the basic cables.
I ordered a E18 yesterday so I'll try using the FiiO one first, if there is one supplied, to see if it can last a while. Would love if FiiO just put out a higher quality short usb cable similar to their L16 & L17 3.5mm cables.
Also, E18 are suddenly sold out recently on most UK sites for some reason, I could only find one place here still selling it (audio sanctuary). Wondering whether if the shortage is to make way for whatever Q-model is replacing it.
Thanks for this orskar, the Pure Silver Cable looks good.