The imp has to be within a WiFi area, so it most likely will be inside something that already has power … an appliance, vending machine, a garage, basement, utility room, etc.
Anything like this would work:
Use a wall-wart for power while you’re developing, and then remove it and switch to battery later on if you wish. There are no rules about how to use the imp. It all depends on what your project is supposed to do. The amp rating of the USB charger needs to be able to handle your load. 500ma to 1amp range is typical.
Note,
You’re not in the U.S., but there are companies like this one (link below) that sell open-sourced imp boards for various uses. Notice on this one how they use a 5VDC 1amp USB charger for power. That’s exactly what I’ve used on some projects: