After checking several knowledgeable sources, I have not found a kit that is approved for the 500B. Bruce has done a few custom installs on 500Bs, but they were field approvals and cost was in the high thousands, probably exceeding $10k in today's world.
So the remaining option is AmSafe airbag belts.
AmSafe has this to say: "AmSafe’s Seatbelt Airbag System (SOARS) is a universally adjustable lapbelt airbag restraint system designed to mitigate head and torso injury in aircraft crash conditions."
Aviation Consumer says: "AmSafe’s website has a number of videos showing deployment of SOARS and that while a double-over-the-shoulder belt system is truly a lifesaver, an airbag is even better at protecting an occupant from flail injuries and hyperextension of the neck."
It's a bolt-in replacement for the lap belt and is approved for all Twin Commanders - even the 700!
Current price is $2580 + installation for two front seats.
https://www.amsafe.com/airbag-systems/soars/
