any one that is 12v and 4.2amps or more should do..... as long as it has the right plug on the end...
ebay is the best bet
check por a little pictyre a bit like a C woth a dot in the middle....... this tells you if the +ve is on the inside or the outside of the plug whigh goes into the monitor
(assuming its one of the standerd little round ones)
8-)