You can probably use the Internet of Things Printer project as a design reference: http://learn.adafruit.com/internet-of-things-printer
The trick there is that we bypass the regulator and feed power from a regulated 5v supply into the Arduino's 5v pin.
Do note the use of the word 'regulated' though.. you want a power supply that puts out nice smooth VCC and GND rails. There are unregulated supplies labeled '5v' that put out roughly enough current to produce 5v across a load with the correct resistance, and there are regulated switching supplies that produce an average of 5v but also include lots of switching noise. The spikes on those can be as large as a volt or two, and that won't be good for your Arduino.
The only supplies I know well enough to recommend are the ones we sell, because I know those have been tested. If you want to try other suppliers, make sure the supply is regulated, and look for information about 'load regulation' or 'ripple'. Be prepared to do some testing on your own with a new and unknown power supply and an oscilloscope to make sure it produces good output when it turns on and off, with a steady load, and when exposed to sudden changes in the load.
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