Ok...so I could not find anything on shortening a speedo cable or on another cable from a different bike that is shorter that would work. I found plenty of shorter cables with a 12mm top nut but all of them had the push in style or clip on style end on the other end. So...I decided i'd shorten mine this way....
I took a butane torch and heated the ferrule on the end where the inner cable doesn't have the "keeper thingy" crimped on it, and basically melted the rubber from the outer housing out from under the ferrule allowing me to slip it off.
I then took my trusty cut-off wheel and cut the housing to my desired length (make sure you remove the inner cable first, you'll need it later).
Now take that ferrule you slipped off and clean it up along with the 12mm nut for it. Take a punch that fits in the crimped end of the ferrule and carefully round it back out until it fits over the outer housing again.
Put the 12mm nut on the outer housing the correct way and then put the ferrule back on with flared end facing out (so it keeps the 12mm nut on the housing). Re-crimp the ferrule onto the housing. (I used a regular wire crimp and a few small punch marks to keep it in place).
For a finished look I used some shrink wrap to cover the ferrule and the outer housing and to help my crimp job and then I used mastic tape wrapped around part of the ferrule down onto the cable to also help and to give the housing better protection.
Now for the inner cable (you saved it, right? This is later...) I ordered this...
http://www.motionpro.com/motorcycle/partno/01-0107/
from motion pro to make the inner cable. Make sure you measure the distance the old inner cable protrudes from the "keeper thingy" and make sure each end is that measurement (about 7/8"). Only one end will get the inner cable "keeper thingy" crimped on it and that end should be the end that is attached directly to the speedometer.
