Newark Electronics stocks Ohmite wirewound rheostats up to 1000 watts.
http://www.ohmite.com/cgi-bin/showpage.cgi?product=rheostats_wirewound http://http://www.newark.com/jsp/brand/brands.jsp?mfg=OHMI
A 12V PWM motor speed control is pretty standard stuff in the RC hobby or amateur robotics field. I have played around with units from IFI Robotics:
http://www.ifirobotics.com/speed-controller-selection-guide.shtml [This message has been edited by NJwirenut (edited 01-22-2007).]
