These drop from regular and advance target dummies, once they explod or out of time. Just loot the remains from them, its a random drop so not everyone will drop a fused wiring. Fuesed wiring also drop from compact harvest reapers, after its time has ran out.
You can find them on the AH, on my server I think they were something around a gold each... (Silverhand) But I am not sure, I bought one for my Mechanical Dragonling then after that did Gnomeregan and got about three. Best bet would be to do a gnomeregan group, but they are semi-rare.
Fused wiring is pretty rare, but as was said above, they'll drop off of your creations sometimes. What bugs me about them is that they don't stack, and I horde them, so they take up a lot of space. :(
it took me 8 target dummies to get 1 of them, but luckily when the dummy extinguishes u get a lot of the parts back anyway so from the 6 i originally had i had enough parts for another 4. it is annoying that they dont stack though, but they are only used in 3 or 4 plans as far as i can remember, the most useful being the jumper cables you just have to be lucky with drops really