jim - sorry to rub it in but I lucked into getting a YZ360B 4.2 kg spring for $24 recently on US Ebay - NOS even.  Very rare.
Gents - dont know whether this will help but Works Performance make an MXB shock that fits the YZB - is the C much different?
"Shock is a Steel Gasser w/triple rate springs, threaded pre-load, 16 position adjustable rebound damping and 1/2 inch length adjustable mounting eye (RHA) @ $534"  This was October 09.  Mate bought one for his YZB and fitted up pretty easy.  I have the contact email address if you need.
Sorry if it doesn't but hope it helps?
regards
Rossco