I know they exist since we have used them in the past, as for where we got them from.. that I am not sure about.
An internet search provided this document:
http://www.rivnut.com/pdf/rivnut/riv...round-body.pdf
Looks like you can get them anywhere from 6-32 all the way up to 1/2-13. Would have to request a quote if going through this manufacturer. I looked around to try and see if there was anywhere else that could provide these but I found what you did, only metric. This is the only site that I saw provided standard hardware.