The only way I know to get original Firman parts is to contact them directly (as I wrote above):
https://rfrcars.com/
However, I think both Ralph and Angela have retired and the company is now run by Ralph Jr. I don't know how responsive they will be. Last couple of times I tried I got no response at all. I would at least try. Back when I was a Firman dealer I used to have a direct contact number and email to both Ralph Sr and Angela.
Honestly, if I still owned a Firman and even if I still had the complete spares package, I would find a source in this country where I could get everything made. The suspension, the uprights, everything that is specifically Firman. This would be the only way I could ensure myself of getting spares when I absolutely needed them. That's what I was looking into before I quit racing.