If you need to make a project portable, the go-to solution is often a 3.7V Lithium Polymer rechargeable battery (often known as a LiPo or Li Po battery).
These batteries have become quite popular - and are used with development boards from WeMos LoLin with their D1 and ESP32 lines, along with others such Adafruit's "FEATHER", Sparkfun and more.
If your product like the ones mentioned above, or anything else that needs powering has a 2.0mm JST connector for LiPo batteries - for example:

... then PMD Way has you covered with a wide range of LiPo batteries that have the matching JST plug - ready to use.
We offer battery capacities from 110 mAh through to 1500 mAh and more.
Unlike other vendors, we don't have any order restrictions. If you'd like two LiPo batteries, or 4297 LiPo batteries - PMD Way can deliver to you for free, worldwide. So click here to visit our LiPo battery range.
And of course, if you need a large quantity, please email support@pmdway.com with your exact requirements so we can organise an excellent price.