Since the device is meant to work with 2 AA batteries, that means it will probably work with a much wider range of voltages.
Fresh alkaline batteries (nominal 1.5v) are about 1.65v and they go down to about 1.35v when empty. Rechargeable batteries are about 1.35v when fully charge, about 1.1v when discharged.
You can also use a regular 5v adapter (usb phone charger or some basic adapter of any kind with around 5v) and lower the voltage the device actually sees by connecting several diodes in series (you can put those diodes in the battery compartment).
For example, a basic
1n4001 to 1n4007 (click link for datasheet) has a forward voltage as high as 1.1v . See figure 4 in the datasheet, you have 0.8v for a current of 0.2A , and about 0.9v for 1A.
So, you could connect 3 of these diodes in series and you'll have a voltage drop of 2.4v, which means that if you use a 5v - 5.5v usb charger/adapter and connect these 3 in series, your device will actually see about 2.5v.. 3v
Other common and cheap diodes are
1n5817..1n5819 (see Figure 2), they have a lower forward voltage of about 0.6v at 0.2A and about 0.8v - 0.9v at 1A. So with 3 of these in series and input of 5v, your device will see about 3.2v at most.
dc in + 5v ----------- [===>|] ---- [===>|] ---- [===>|] ------- + (about 3v)
dc in - gnd ------------------------------------------------------------ -