Im sure you can get a power plug from electrical stores ( im from aus so no idea what yours would be )
And yes polarity should matter - from memory the inner of the plug is positive ( + ) but just check on the AC adapter - it will tell you... if not, measure it with a multimeter
then you can connect it to your 1900 - molex plug or whatever should be fine ( correct me anyone if im wrong

) - just get a normal extension molex adapter and butcher it to get the 12v rail...