Watts is Volts times Amps
Doing arithmetic, it follows that Amps is Watts divided by Volts.
In this case, 5/12, or 0.42A, or 420mA.
More important, is it a transformer or a power unit or a power unit badly named as a transformer? A transformer on its own gives out AC, a power unit gives out DC. There might be a clue on the the thing it plugs into - if it has a "+" and a "-" by the socket, it wants DC. 420mA is usually just that bit too much for one of those cheap battery eliminators. One of these
http://www.ebay.co.uk/bhp/12v-500ma-power-supply might do the job, but might equally need the plug sorting.