jonke...I got a Ti RCS (Renton Coil Spring) 450 x 2.8 that I am about to sell, which should fit. It has just been taken off a Roco RC WC 215 X 63mm shock. It has an inside dia. of 36.3mm which is the correct size in/dia Cane Creek quote for the Double Barrel. Most springs out there are 35-6mm in/dia, and although some will fit the CCDB, they may bind or rub on the on the plastic wear sleeve (which I know is what its there for) not only effecting shock performance and wearing the inside of the spring, but worst of all it makes a bloody annoying noise. Which I found out when I fitted a Nuke Proof Ti spring to my CCDB.
Anyhow, the spring is pictured below, and along side a Marz 500 x 2.5 spring for length comparison. marz free length 146mm, RCS 151MM.
These retail for about £200, I want £95 (1st class rec.) If your interested. Good luck if your not