In fact, the basic sources used by etymonline.com are mentioned on the site's home page, which also links to this more extensive (complete?) list of primary and

In fact, the basic sources used by etymonline.com are mentioned on the site's home page, which also links to this more extensive (complete?) list of primary and secondary sources: https://www.etymonline.com/columns/post/sources https://www.etymonline.com/columns/post/sources A few of the individual entries do reference one or more of these sources by name; for example, the entry for "better" says "...Boutkan finds no good IE etymology", where "Boutkan" is one of the entries under "Other Sources".