What application.
the monitor should have a 5 digit number proceeded by a letter
ie A58764
The generic answer would be Radio Shack. They have a universal 12volt supply then get the correct plug out of the hundreds they carry.
I wouldnt worry about it being more amperage just make sure its the right voltage.
also are you sure its 12volt.
You can get monitor power cables at most stores, even walmart should carry that sort of thing. Of course if you buy online you'd pay a bit less, but it depends on how fast you need it.