Try these guys:http://www.pegasusautoracing.com/group. ... WARNSWITCH
You probably want to get a higher pressure switch than the ones you get at the local parts store - ideally one that goes on just a few PSI under whatever your normal hot running pressure is. This way you have some warning that pressure is dropping - a normal 5 PSI switch is going to light up just about when you motor is seizing. Typically these switches mount using a standard pipe thread - you can get the tap at Home Depot and tap out your existing sensor port. (though you'll probably discover it's already 1/8 NPT)
You can also replace the pressure line to the gauge you have with plastic covered with braided stainless. if it's protected where it entered the engine and gauge this should be as reliable as brake lines are.