If you want to find the DOI for an article or ebook chapter, Crossref provides a free DOI search service. Search using bibliographic data like title and author.
Search the free DOI Lookup site at Crossref.
The DOI Resolver browser extension does exactly what its name suggests. Often on publisher sites and institutional repository pages, Digital Object Identifiers (DOIs) are shown (such as 10.1016/S0921-4526(00)00753-5) but aren't actual links. DOI Resolver allows you to highlight the DOI, right-click, and select "Resolve DOI" to be taken directly to the item. Also, clicking on the DOI Resolver icon opens a DOI search. One useful feature is the extension's ability to generate a QR code for the DOI.