It's not a general feature for each gamepad - it depends on the driver. If you're using a Dualshock 3/4 gamepad, then this information can be found by querying the /sysfs- see for example https://retropie.org.uk/forum/topic/12945/wireless-battery-indicator-ps3.

Pololu are good for voltage regulators

This one can make 5v from a massive range of input voltages
https://www.pololu.com/product/2836

Tend to be pretty tiny too