Guy,
First you have very little chance of finding parts for an old boat that has been out of production for 25 years, whether the builder is still in business or not. Fortunately, I wouldn't think the mast boot would be a Cal part anyway - but even if it were - you can find a number of kits for fabricating a boot. You can get a platic boot kit to cut to fit, try self-adhesive boot tape, or the ultimate solution
http://www.spartite.com/Default.aspx?SiteID=3
You might try the Cal email list to find out what other 30 owners have used.