What batteries are used in Fitness Tracker Bands and where can I buy them?

To my dismay, I havent been able to find the type of batteries/ the name of the kind used inside a smart watch/fitness tracker/e-band/Xiaomi Mi Band etc.

I am working on a bluetooh arduino project that heavily relies on the smallest probable size of the power source as well as Longevity. And only the battery life of the ones used in the above mentioned devices comes close.

It is much appreiated if anyone could point me to the type/name/link of such batteries and also where I could possibly purchase them(favourably online)

Thanks in Advance!

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