Welcome to the forum.
The answer depends a little bit on what you want to achieve. If you do not care about power use a USB power adapter and connect your board via a USB cable. The same is true for USB power packs but you will have a time limit depended on the size of the pack maybe a day or two for an average pack.
If you care about power and want to run the board as long as possible from a battery you need to first make sure the sketch uses as little power as possible. There where are few discussions in this sub forum (disabling LEDs, sleep modes ...). Have a look. Also check the BLE Sense sub forum.
Then you need a 3V power source and disable the onboard power regulator by cutting the bridge under the PCB (see FAQ in store link below) . Then you can power the system via the battery directly.
BLE was designed for 3V coin cells like the CR2030. That is enough to runs small sensors for many months e.g., heart rate monitors, cycling sensors, temperature sensors ...
Unfortunately, the battery you have has the wrong voltage and therefore will always require some voltage regulation which will burn more energy than using a 3V battery in the first place. But if you only need to run the app for a day or a few, you could try to build your own little regulator circuit and then bypass the onboard regulator. The onboard step-down converter has likely been chosen because of the large input voltage range. You can do better because you do not need that.
There are many options, I suggest you use a parametric search on DigiKey or another distributor.