Unfortunately, Benjamin-Sheridan obsoleted the Equalizer years ago, and liquidated their remaining stock of parts and spares. They officially will no longer service them anymore.
Both the rectangular body seal and the reg seal, I've made, in the past, from sheet urethane that I got from either MSC or McMaster-Carr. I honestly can't recall the durometer or thickness, it's been close to ten years now.
I did once, however, punch a quick-and-dirty (but functional) reg seat for an Equalizer-style reg (probably a KC NoHotShots) using just a Whitney hand punch to pop a disc of approximate size out of an old blue Automag bolt bumper.
