What I'm trying to find is an additional safety plunger for an extra slide I have as so to share the frame w/2 slides (4.25" & 5" ported core models in 9mm 1.0). Is there such a safety plunger ONLY option to buy or .................

If you mean the striker block, then yes, you can buy just the block and spring from various places like Brownell's, Midway etc. You can get a better one like the APEX Tactical from APEX and also places like Brownells, Midway and other sources without buying the entire slide or trigger package.

APEX TACTICAL SPECIALTIES INC. S&W M&P ULTIMATE STRIKER BLOCK KIT

Extended Striker Block Plunger for M&P(R) (does not include spring)