• Try narrowing your Google search by site:jstor.org, site:academia.edu; site:archive.org
• Think of related search terms (like Library of Congress subject headings). Here that might be candy/sweets/junk food, or s'mores/fluffernutter/moon pie
• What do you already know about marshmallow? That should give you some other leads. Ingredients? Inspiration? Look those up!
• Footnotes in the sources you already have
• If there's a foundational foreign term--if the thing in question wasn't invented in an Anglophone country, say--try looking up that language's phrase on its own Google (you can use Google Translate once you get results)

Basically--get creative with search terms, and be ready to use in-person sources as well as online ones.

Neat topic! Good luck!