Plenty about on Ebay. I bought a Pioneer DEH-5000UB brand new for £25, really nice deck with USB, MP3 player, etc.
Like this one;
http://cgi.ebay.co.uk/Pioneer-DEH-5000UB-In-car-CD-MP3-USB-Radio_W0QQitemZ110284400180QQcmdZViewItem?hash=item110284400180&_trkparms=39%3A1%7C66%3A2%7C65%3A1%7C240%3A1318&_trksid=p3286.c0.m14
but cheaper
Like this one;
http://cgi.ebay.co.uk/Pioneer-DEH-5000UB-In-car-CD-MP3-USB-Radio_W0QQitemZ110284400180QQcmdZViewItem?hash=item110284400180&_trkparms=39%3A1%7C66%3A2%7C65%3A1%7C240%3A1318&_trksid=p3286.c0.m14
but cheaper