Are there spare parts, such as a valve seal or an o-ring, available for the old Vieser air admittance valve?

The Vieser air admittance valve is meant to be used in connection with a vent pipe that vents the air directly outdoors. Where the local sewer works so permit, venting can also be arranged using only an air admittance valve. The air valve must be located so that there is no risk of freezing. The valve must also be located higher than the water level in any of the connected plumbing fixtures. The air valve must be installed in a place where air is readily available.