I have one of the Wico imperial PS2 trackballs, they are spiffy.
I bought mine on ebay, but I seem to recall them only being $80 from Wico, not too bad considering that you don't need an interface.
Also, look harder on ebay, the guy that used to sell them before always listed them in the wrong category, they were never in the arcade category, usually in the playstation or console games category, or something like that.