Commercial Pattern Archive
This Commercial Pattern Archive (CoPA) has over 50,000 commercially published clothing patterns that originally appeared from 1847-present. Unlike most of the electronic resources provided by UCI Libraries, this DOES NOT require VPN. It does require a specific username and password that you can obtain by logging in here with your UCINet ID and password: https://www.lib.uci.edu/online/copa Once you've obtained this information for CoPA, continue the login process using the links on the screen, manually enter the UN/PW, and then search for what you want.
HINT: A helpful COMPLETE SEARCH might be choosing "Hat" in the Garment type and combine it with one or more relevant years.
Other Resources
ARTstor and AP Images would both be great resources to find pictures of hats. Links for these resources (and other great image-focused databases) can be found on the Design page.
Books
Encyclopedia/Book Chapters
Bloomsbury Fashion Central is a good online resources that has many different articles/book chapters related to clothing in one online portal. It can be hit or miss on whether you find something appropriate to your topic, but there are many different materials focused on hats/head coverings throughout history and from different societal groups. This is a basic search that attempts to bring all of these articles together. Specific examples include:
Some great places to find pictures of historical hats are in historical magazines. Here's a few that you should make sure to check out:
Selected volumes from the library's collection:
Women's headdress and hairstyles in England from AD 600 to the present day by 
Nineteenth Century Fashion in Detail by 
Victorian Fashion Accessories by 
Fashioning the Victorians by 
Edwardian Ladies' Hat Fashions by 
Wartime fashion : from haute couture to homemade, 1939-1945 by 
Fashion in the Forties and Fifties by 
