Apple hasn't included power adapters with their devices (except computers) for quite some time. Apple was getting penalized in some countries because the chargers were being dumped in landfills. You can use any charger or computer that has a USB-C port. Here is one Apple sells:
Buy 20W USB-C Power Adapter - Apple
There are many other third-party adapters as well. If you have an old watch that has the older USB configuration, you can use that as well. You will simply lose the fast charging feature. Since I charge mine overnight, every night it doesn't matter to me.