Purchasing a complete power supply is likely to be the best approach. The one that Amri points out is often called a 'wall wart' - probably because of the way it apears on a wall when plugged into an outlet.
What you should do is determine, as best you can, the requirements of your power supply in terms if volts (min, max), and current (usually max is enough). Quite often an item that requires 12 VDC will run acceptably if the voltage varies by a few volts either way though some things require a tighter tolerance. The power supply current capability must be equal to or in excess of your requirements with one exception - the 12 volt wiring and the device you are powering should be protected from over-current. The supply might be capable of delivering more current than the wire or load can handle.
There are other issues that might be of concern - ripple or noise are somewhat fast variations in the voltage. Regulation describes the changes in voltage that result from changes in the load or the source. Ripple, noise and regulation are usually very important when directly powering digital/computer circuits and are addressed in the better and more expensive supplies.
You can purchase wall wart type supplies that will deliver 12 volts but be careful that you get one for DC (I have some at home that are AC) and that the connector is properly polarized. Some might be regulated and well filtered and some may not. You can also purchase low cost bench top supplies - good for about 2 amp. Wall warts seem to cover an amp or less. I just purchased some supplies that had 4 or 5 different voltages up to 12 VDC, regulated, 2 amps for $25 (US) each. They are self contained with overcurrent protection on the output.
Hope this helps.