Quoted from o-din:
Bells sound much better, believe me. And they are available from PBresource except for the 5 inch one.
The 3" and 5" are available now from Pinball Resource as complete assemblies:
However, 4" bells have been unavailable for quite a while. A person could use a 3" plated bell for one tone and a 3" black powder coated bell for a different tone in place of a 4" bell.
Naturally, you would need a Stern 50 volt coil to be installed in these bell assemblies.