I think my last NO magnetic contacts came from Radio Shack. http://www.smarthome.com carries a full line of magnetic switches. Part # 7040 is the NO variant. Any online site that carries security devices will probably have magnetic switches of the NO and NC type. Retailers like Lowes or Home Depot may also have them if they carry security devices.
You have a Normally Closed (NC) magnetic switch if the Input is ON, GND connected to I1- through the magnetic switch, when the magnet and the switch are adjacent. Removing the magnet will open a NC switch, turning the Input off, because the GND is no longer connected to I1-. A Normally Open (NO) magnetic switch works just the opposite. When the magnet is adjacent to the switch, the switch is open. When the magnet is moved away from the switch it closes.