I bought the Birdcord and a Anker Powercore 13000 and they works perfectly with my Digitakt and Digitone.
Birdcord is a good option and you can use with different powerbank. Generally speaking, every powerbank with an output current of 2.1A or higher is good.
The mAh-rating tells you how much electric charge the battery can hold. The higher the rating, the longer it gives you power. A 10000mAh powerbank will power the Digitakt for roughly 4 hours.
Don’t forget the 2.5mm Adapter.
Hope that this could help!