You can find a water bottle cap replacement by searching for your water bottle brand and the term "replacement cap" on Amazon.com. For example, in order to find a replacement cap for a Nalgene water bottle, you enter the search term "Nalgene replacement cap" in the search text box.
Depending on the water bottle brand, you may find a replacement cap on the company's website. For Nalgene bottles, you can navigate to nalgene.com, click on the Shop option at the top of the page and select Replacement Caps in the left sidebar. For Klean Kanteen stainless steel bottles, you can click on the Shop Klean menu on the company's website, kleankanteen.com, and select Caps & Lids.