You need to contact the manufacture of the tub or its local supplier. They will be able to get these for you. There is from what I know no standard over the counter replacement parts available.
Do you know who made the tub?
20 year old house......there are no markings of the manufacturer!
Check the faucets, That may offer some clue. If you can get to the motor area there should be some information in that area also.
If all else fails, call a marble supplier, or a local business that offers and installs marble tubs, they should be able to help.