Try a Cannondale dealer first. I don't know if the company has any obligation to support 1988 product, especially after the bankruptcy, but worth a shot. Should be the same item for many and several years of production. Might be in a junk drawer--saved from a frame that went in for warranty service.
If you're skunked, no shame in using a 31.8 mm clamp=>braze-on mount. Basically, it's a clamp with provision for fitting your bz derailleur. Shimano have made them, as have some others. You should be able to relieve the mount in the area of the fittings, as the Shimano units I've seen had lots of material, but of course it's been a while for me.
Good luck
Mel