But the question is - is there anything you are using that needs 12 V. If not, then a 12 V supply is the wrong supply!
USB "phone chargers" are readily available having a regulated 5 V output and those for the iPad and similar tablet are generally rated at 2.1 Amps.
If you connect them to the USB socket on the UNO, you can draw from the "5V" pin (with all grounds connected of course) about 500 mA minus what the UNO (and everything connected to its output pins) takes.
It is better to connect your 5 V to all the devices requiring 5 V and the "5V" pin to power the UNO itself, but you must disconnect the 5 V supply from the "5V" pin whenever you need to plug the USB into your PC for programming or serial communication. 