I was hunting for a Ksyrium SSC SL spoke last year but couldn't remember the year I bought the wheels ; if you know the year then either Mavic or a bike shop that sold these wheels can tell you the Mavic part number for the spoke ; I ended up measuring my broken spoke and found a bike shop here in Connecticut that still had some of them - as you know the older ones are out of production - as it turned out I think mine was 2006 with a part number of 32418201 ( Ksyrium SSC SL, silver, milled rim, rear, drive side, radial spoke)
I've attached info that I found online from the "Rogue Mechanic" and the bike shop here in Ct is :
Target Training
772 Post Road East
Westport, CT
phone = 203-227-6177
If you find a bike shop out of your area that has these and you don't know the year , then it's very important to measure the spoke very accurately in millimeters so that the shop has the correct spoke .
Good Luck !!!!