I can't think of anyone who might offer more than the two common styles -- one has a round bar for a flip-piece and the other has a flat sided piece that tapers toward each end. Most will probably carry just one. Rio Grande or Stuller would be good choices.

Personally, I think you can achieve a much nicer pair by designing your own backs. That used to be much more common than using a finding. I just posted a pair I made some time ago here. You can find examples of many more styles by searching the web for "antique cufflinks," or "estate cufflinks."  

I think the one piece ones are nice. Rio used to carry them but I haven't made any for so long that they may not be available there anymore.

marilyn