Grab the standalone singlefile executable following this procedure: https://github.com/gildas-lormeau/SingleFile/blob/master/cli/README.MD
Clone this repo, then load the el file.
Set your desired location for the stored webpages with the variable singlefile-destination-dir
.
Call the function singlefile-bibtex
and give it the URL you want to save and add to your bib file.