Just buy a piece of battery replacement, open up the controller (there are many youtube videos) and swap it yourself. Battery is the easiest to swap cos it's the first part inside the controller that you can access. (To replace the sticks, for example, requires you to first remove the battery, then some cover, a pcb board etc.) Just need to be careful with the shoulder/trigger buttons.
You can buy batteries online from Aliexpress etc. Prices seem to have gone up, last time I bought it was under us$5, now us$6-10. (btw, better dont trust the battery capacity that they quote.)