How to buy a iPhone USB cable

You can buy cables from apple:

USB-A: https://www.apple.com/shop/product/MXLY2AM/A/

USB-C: https://www.apple.com/shop/product/MX0K2AM/A/

(Depends on your power adapter)

You can also buy third party cables and Apple may replace them, but they might have stopped doing this.

You will need to purchase a replacement cable. They can be found at most department stores, or electronic retailers.