Sweat caps are what you mentioned. They are the end caps used to block off copper pipe in the plumbing or refg. trade usually available at your local hardware. Sometimes if they don't have them in stock they can order them. Paul Daigle and many others feel the same way you do. Chromed Sweat caps were used for his accordions.

I like the metal buttons. I have them on my Bon Cajun key of A accordion. I like the feel, and the look is good too, but the aesthetics are a secondary issue. I wouldn't say that I have a gigantic preference for metal over plastic. For example, the accordion I play the most is my Master Bb which has plastic buttons. I don't feel a huge motivation to go out and get them changed to metal, but it is a thought. However, at the rate I go with some things it will probably never happen, especially now that they are becoming scarce. I would be a shame if a replacement supplier for them could not be found.

Oh, one more thing. A long, long time ago (1990), I was in Quebec and was jamming with some guys. One of them had an Acadian with metal buttons. He let me play his accordion, which was the first time I had a real LA Cajun accordion in my hands (I had only played HA-114s up to that point). Those particular buttons were very different from others I have seen since then in that they were somewhat small in diameter and quite rounded on the top. All the others I have seen since then are much flatter or even slightly concave. I would definitely not want those rounded type of buttons. They were too rounded and felt kinda weird.

-David