You can find them on Ebay for cheap. Something between 30-60€ and NIB for just a bit more like this one:
NIB Microsoft Optical TrackballI like them both but I think I prefer the Trackman wheel because of the size, I also have them both but my Microsoft is a bit scratchy.
I have yet to try the Microsoft Trackball Explorer and I think I reeally will enjoy it but they're too expensive.