I have worked on a few of these, and it looks like you have one of the cooktops that you need to take off all of the burner bases with a socket wrench, and remove all of the knobs. (Sometimes the burner base bolts are very hard to remove. I have had to carefully heat them up with my torch in order to get them off. A pain in the neck.) The potentiometer is on the burner valve. I am sure you will recognize it when you open it up.
Always make sure you unplug it before working on it because when you go to put the cooktop surface back on, you may push down on a burner valve activating the spark module and you will get a good zap.
You should be able to buy the part from the following links http://www.coastparts.com/mfgs.asp