If it's an original C12, the clamp-on shockmount manufactured by FLEA
should fit. It's about 135 Euros. It's a beautiful piece, totally fitting for a fine C12.
The FLEA C12 is manufactured to original mechanical specs.
Another possibility is the Rycote USM
. It will handle mics up to 900g weight and 68 mm in diameter (VB version), so should fit a C12. They list the C12 on the Rycote compatibility chart as fitting the standard (55mm) USM.
FLEA C12 shockmount and Rycote USM pics attached