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