According to this summary of NEC requirements for radio antenna grounding (from 2014, but hopefully still somewhat accurate):
It says "Antenna lead-in surge protectors must be listed". As far as I can tell, there are no "listed" SO-239 lightning arresters available on the market.
I even contacted Alpha Delta to ask if theirs were listed, and they said they used to be, but no longer are because their customers did not require it, and the certification was expensive. I looked at other brands online, and none claim to be listed.
In order to be compliant with NEC requirements, where can an amateur radio operator get listed SO-239 lightning arresters? Thanks.