I want to buy a new band for my Apple Watch

https://www.apple.com/shop/watch/bands

This is the official page for Apple Watch Bands.

Note: this page is only for the US, if you want to navigate to your specific country, go to https://www.apple.com/choose-country-region/, choose your country or region, and then enter the main page of the website. Scroll down to the bottom of the page and then click "accessories", (should be the 10th link below Shop and Learn). Once you enter that page, click Apple Watch Bands under Browse by Category, it should be the first circle on the right.

You can also go to the Watch section, on the top of the Apple Home Page, which should be the fifth link, and click bands right underneath it.