Parts Connexion up in Canada has many nice RCAs. I really like the Cardas, but you have quite a few to choose from:
Check also with Michael Percy Audio. He sells all the Cardas variants plus Vampire. Also, I bought some that are either made by Eichmann for their bullet plugs or are sanctioned for use with them; maybe from Percy or some other US source. It should be no problem to find high quality "female chassis mount RCA jacks". (That's what you are looking for.)
Lewm: Have you ever done a dstructive test to see what the material is? Most likely it could be brass, tinned brass.
I don't have any extras to wreck, but if I find one I'll post what I find. Gold is good enough for so many things, and the primary concern is corrosion resistance. The thickness on all connectors is quite thin, but it is effective. Also, what structural metal do you consider "best?" I know silver is the best, but not everything is made with silver. SMT components with silver connectors? I doubt it. Radial or axial leads on resistors & capacitors, I doubt it. So, take one part in the electronic chain made out of a silver wire & all the others out of copper or brass alloy kind of negates the benefit of the silver doesn't it? Just asking...
Not that I can intelligently debate the issue, but I had come to like the Cardas female RCA jacks that are brass under silver under rhodium plate, rather than gold.
But since then, I also like the less massive female RCAs that were sold to me when I asked the question "What female chassis mount connector goes best with an Eichmann bullet plug?" Without question, my favorite male RCA plug would be the Eichmann silver bullet (copper, too). I am not entirely sure that Eichmann makes the female jack that was sent to me, but it does work well with the Eichmann male plug. (There were four in a box set, but I discarded the box before noting the brand.) I cannot tell what they are plated with, does not look like gold or rhodium. Might be something over copper. I mentioned Vampire brand; those look good to me too and are cost-effective. The ones sent to me for use with the Eichmann may well be made by Vampire.
I've kind of made a decision not to fret over RCA plugs any longer.
Rolling the dice in darkness...to enlighten the dark cave:
AES Standard 100: copper, purest
AES 94: Gold (6% loss)
AES 106: silver, technical (12% better than gold
AES Brass: very low, something to avoid, is used for cheating high quality (thin layers of gold onto), maximum profit, nice optics, typical audiophile product
A good store: Partsconnexxion