Starting with a browser like Google or Duck Duck, put in terms like masks 1918, protective masks 1918, history masks transmission, etc. In addition to a number of websites and magazine or newspaper articles chronicling the wearing of masks in earlier epidemics or the 1918 Pandemic, the category, Images, is useful for finding illustrated sources. Google Images contains images like these from a Berkeley newspaper and a Milwaukee Anti-Tuberculosis (and Flu) campaign.

For national or international news items regarding the wearing, production or, say civil liberties with masks in 1918, some of the most useful databases may be the Proquest suite of newspapers, especially the New York Times, the Chicago Tribune, and, the Los Angeles Times, giving you a broad regional range of commentaries.
 

Many of the ordinances, like this one in San Francisco came with sanctions.
 

Photographic archives, like The Influenza Encyclopedia from the University of Michigan, or the Images from the History of Medicine Collection from the National Library of Medicine are particularly useful for images and background information, including the wearing of masks and other hygiene recommendations during the 1918 Pandemic. 

For a more public health rendition, try PubMed, IndexCat (late 1800s to 1940s) and CINAHL which index clinical, medical and nursing journals, books and pamphlets. Some of those materials will be available as full text online, some in restricted collections in campus libraries, either due to COVID-19 restrictions or to their age (pre 1923 may have restricted access). Using the terms, like those in a browser search, like masks...will retrieve articles like: The History and Value of Masks, by Christiane Mtuschek, et al

Remember that books and journal articles have footnotes or references that may lead you to additional sources.
 

Books (and some journals) written over the last 100 years may have chapters or illustrations about the use of masks during epidemics or pandemics. Google Books has a nice feature, click on Tools, and see the Any Time tab. You can limit your search by various date ranges, including a Custom Range, like 1918-1920, which will get you something like this Journal of the American Medical Association article on the proper material for a 1918 mask. Note: your search may get complicated by discussions on gas masks, also a “thing” in 1918, due to the war effort.