There are a few decent/cheap ones on Amazon now. When I bought mine three years ago there were very few out.
If you just want to get this installed and don't plan on ever using it again you could use this DIY. I made a similar design, but I used a full thread allen bolt with a nut and a
thrust bearing . Held the allen bolt and tighten the nut with the thrust bearing between the nut and the rivet nut. Either way will get it done.
Dorman sells them as well.
https://www.spyderindustries.com/blogs/news/diy-how-to-install-nut-inserts-rivet-nuts