They're not too rare. I know where there's about five clusters with the 120 speedo but the owner won't sell just the speedo. They show up on ebay now and then. You can probably get a whole cluster for $30. to $50.
Man, I was hoping they were kinda rare...lol, I have one I found at the junk yard in excellent condition that I don't need because I'm going with all aftermarket gauges. Oh well, I guess I could always try to ebay it and get something for it.