Jim all the ones I've seen on PGR rides are on a luggage rack. And every one of them that I asked where they got it, they either said they made it or a welder fabricator made it for them.
They mostly are taking two flat straps (stainless) and drilling a attachment bolt hole thru both and hold it on the luggage rack. They then weld a flag mount on the top strap.
If you don't have a Sissy bar or luggage rack I have not yet seen what they do on that, but I will check now with my stepson who has been on many more PGR rides than me.