Catalina 30 Parts
Try Catalina Direct they are a large Catalina distributor in Sacramento CA. or you can call Catalina Yachts, they are in Woodland Hills, CA and have a parts department that deals with owners. Most boats have a round waterproof hatch over end of rudder shaft, this is fairly common part and either of the above should be able to help.
On Catalina''s site you can e-mail parts department under "contact" or phone them, number is at bottom of page.