All you need is a battery big enough to power it for a couple hours.
This is a 3 Amp-Hour portable with carrying case. I recently picked one like this up at BJ's Wholesale for about $25, and they're getting more popular in the market, so you could probably find one nearby.
Doug Burnside (Tanstaafl) used a small battery like this to power his Bikempeg, but his was 5 Amp-hours. He said he could get 8 empeg-hours out of it without Deep-cycling the battery.
Once you have the battery, you just need a backpack/briefcase/something big enough to fit the Empeg and battery, and then a pair of low-impedence earphones with an RCA converter.
Keep in mind though, it's gonna be
heavy.