Just the processor? New? To be honest this kind of stuff is obsolete these days - you can pick up Dell P3-933 system boxes for about �150, so I reckon a 600Mhz proc can't be more than about �50.
Ebay or computer fairs are your best bet. Slot PIIIs are more expensive than sockets ones and the 100MHz FSB are the most expensive (I saw a Slot 1,PIII 600 MHz, 100 MHz FSB last week for �45).