battery backup for USB devices

I bought a Garmin Edge 305 this summer. It's a nice machine but the internal battery must be recharged before each long ride. One day I forgot and it simply quit. On a camping trip or multiday bike trip you may not have access to A/C. The solution is straight forward - have a portable power supply (4 AA batteries) with a USB plug. You can find them on the Web but I'm looking for a supplier in Toronto. (Curiously, neither Best Buy nor Future Shop carry this device - only the near equivalent which is one powered from a car battery accessory jack). Does anyone know where you can get a portable USB power supply in Toronto?