The riv-nuts are available thru Aircraft Spruce,
www.aircraftspruce.com, amoung other aviation hardware dealers. They may be available on ebay also, but I have not checked. I did this job over 20 years ago, but it wasn't unsurmountable, and I don't remember who I bought the parts from, but it probably was a company like Van Dusen or Aviall, as those were my primary hardware type vendors. Van Dusen has been bought out by Aviall since then.