Replacing a 5v to 3.3v converter
The 2nd Generation (2012) Airport Express uses just a 3.3v internal Supply. Unlike the 1st generation, it uses an on-board boost/step-up converter to power the USB port at the typical 500~700mA.

It comes with a Delta branded 120v AC to 3.3v 2.09 Amp power supply. Of course, not all of this is used, and the most draw will come in when all the features are used (Both ethernet ports, optical out, usb power, etc).

Unfortunately, noone has bothered to hook up an ammeter to measure what the typical or max current from the supply is.

The one recommended in the article is a 800mA max Linear Regulator. You will need a usb supply that can produce atleast 700~800mA 5v, and then a linear regulator to step down 5v to 3.3v atleast 800mA. Most 3.3v linear regulators can do this, even the ones at radioshack. Look for a 1Amp one. Make sure it has a heatsink at the very least.

Or do the smarter thing and go for a 1 to 2 Amp 12v to 3.3v regulator. You will end up paying the same. A switching regulator has little heat issues to worry about.

KEEP IN MIND that that 800mA they used, was only for wifi and 3.5mm out. Do not plug in a usb device or ethernet device, or you might have issues, including a dead Airport Express.