Have you checked with Precision Scale (PSC)? PSC owns the Kemtron line of parts. I model in On3 and I know they have a lot of the Kemtron On3 C-16 parts available-- I would think they would have the HOn3 parts available too. If you order frames from them, they will probably come as un-machined castings. I would also suggest calling Stan or Sheldon Schwedler at Coronado Scale Models in Phoenix. They seem to have a way of getting those parts and may be able to order what you need from PSC, if they don't have them in stock. Good luck.
Bill M.