Source of little

[Roboteernat]

Although you say ready made, if you want to make your own, typically a 330 ohm resistor will be suitable with most LEDs connected to your supply link with 4s.

If you want to be specific you can find the forward voltage and current on an led and calculate, and this will be a heck of a lot cheaper too.
https://www.digikey.co.uk/en/resourc...eries-resistor

For blue or white LED's they usually have higher forward voltages but 330 - 390 ohm resistor will be good.