Which adapter do I buy for Apple Watch se, please

Any 5W or better USB charger with a matching plug (USB-A) will work, if that’s your question.

Here’s one: Apple 5W USB Power Adapter - Apple

If you have a USB-A power adapter for an iPhone, that’ll work, too.

Here’s the USB-A cable that’s in common use, and the cable that’s been provided with most (all?) Watch purchases:

https://www.apple.com/shop/product/MX2E2AM/A/apple-watch-magnetic-charging-cable-1m

Apple now has a USB-C cable, as well. I don’t think this is the one included in the Watch boxes (yet).

https://www.apple.com/shop/product/MLWJ3AM/A/apple-watch-magnetic-fast-charger-to-usb-c-cable-1-m

That second cable will need a different (USB-C) power adapter.

If you have a computer with USB-A ports (and the USB-A cable), that’ll usually also work.