Seems simple to me to get them from Bill Mitchell. I've just spent the last 6 minutes looking (for me that's a lifetime of looking online) and while they are out there, they're really not easy to find. Also they look like plumbing parts, not racecar parts.

How does the plug seal? NPS threads need some sort of seal. One type I found has a hex head with an o-ring under it. Also not cheap at several dollars each.

R.