I know that Kohler stopped selling these toilets in 1996 when the manufacturer went out of business. Kohler says that no parts are available. Does anyone know of a third party manufacturer making parts that will work?

I don't think you will find anyone that will sell you parts for this commode, the reason being that the commode is very dangerous as far as damage to your home is concerned, they have been known to (explode) and do a lot of flooding damage. I think you would be money ahead to change out the commode.