To power LEDs, it is important to have enough power to the shield. The voltage is really depends on number of LED or output voltage. Each LED has about 2-3.5V forward drop voltage. Supplying voltage to the shield should be higher than total voltage drop of LEDs.
Output Amp is also important. For 350mA LEDs, about >1.2A per shield is good enough. For 700mA LEDs, >2A is required.
For general rule of thumb, please check the manual (Input voltage and operation frequency section).
Most common power adaptors are 12V and 24V. They are typically designed for laptop computer power supplies which have normally have >2A output Amp.
They are available in many different places including local electronic stores. Here are few online places.
As long as adaptor has 5.5mm(outside diameter) and 2.1mm(pole diameter), adaptor will fit with the shield.