Don't like the standard shapes of the common mice on the market today? You could always carve your own.
Make has the details on making your own wooden mouse. It's something of a cheat, because it relies on taking an existing mouse and gutting it for parts, which will also somewhat determine its absolute final shape. Still, if you wanted a mouse for a particular style, ergonomic purpose or just because you like carving, it's a solid project to take on.
Build a Working Wooden Mouse [Make]