Much will depend on the following...
The power consumption
The connector style for the DC
Where you live
You might seek out a direct identical replacement but for that you will need to know the make and model of the set
Alternatively you can use hobby electronic shops - yellow pages - Amazon - e-bay
In the UK Maplin have a range of PSUs - you might get away with a voltage "close" to 12v rather than 12v exactly - as long as the power rating is sufficient - same or more but not less than.