You need a source of DC power that will connect to the motor shield and be a source of the DCC signal that will be sent to the track. The motor shield is rated at 2Amps but the code for the DCC++ base station limits the current to about 800mA. This can be adjusted by making a change in the code. If you want to drive the motor shield to its full capacity at 2A you may want to add some form of cooling to the chip on the motor shield.
In any case, you need a DC source of at least 1A and about 15 or so volts. This will result in a track voltage of about 14.5 volts. A good source of clean DC power is a laptop power supply. They are a switching powersupply so provide good, regulated power to the motor shield.
Do not be tempted to use a DC power pack as that DC voltage is not regulated nor is it clean. Assuming it is a 12V power pack, it doesn’t have enough voltage to properly produce a good DCC signal.
Here are some possible power supplies: