I was using a vertical Logitech mouse but was still experiencing discomfort mousing out to the side of my enormous Advantage2 keyboard. In an effort to head off any compounding RSI issues, I switched to a ploopy Adept trackball. Had no idea if I'd like switching to a trackball, and I love it! My hand never moves in an awkward motion, and mousing has become pretty restful. Better be careful tossing the ball up in the air idly during meetings as I won't be able to get a replacement if ploopy has paused shipping to the US (It's a pretty dumb habit at any rate).

They're just 1.75" snooker balls. It's very easy to get a big box of them. You'll also quickly notice why red never goes out of stock on Ploopy's website.