They don't use a conventional lip type seal. Under the bonnet is a spring loaded packing gland (In addition to the valve plug spring). Replace the packing with a graphite type rope packing.
Were would i get the packing?
Used to be that pump suppliers would have an assortment of packing. String packing is still available at most plumbing supplies. Might look into McMaster-Carr industrial supplier. If it is a square cross section packing you will need to get dimensions.
I don't think it will do much good but the LaPlant-Choate seal assembly part number for the control valve is 19899.
Have no idea where you might source it.