Reference resources are a great place to begin your research. They can help you define terms as well as to find background information about a topic, a chronicle of its history, theories, key people and sometimes a bibliography of additional sources.
There are many different types of reference sources, which include dictionaries, encyclopedias, thesauri, bibliographies, handbooks, style manuals to name a few.
The Berg Companion to Fashion
A Cultural History of Dress and Fashion
Encyclopedia of Clothing and Fashion
The Greenwood Encyclopedia of Clothing Through World History [3 Volumes]
Survey of Historic CostumeAn archive of Vogue magazine (US edition) in full color page image format, from the first issue in 1892 to the latest issues.
Oxford Bibliographies offers peer-reviewed annotated bibliographies which will help you to narrow the vast number of citations to the best scholarship on a given topic.
Bibliographies are browseable by subject area and keyword searchable. For example, there is a bibliography on Costume (Renaissance), Dress (Medieval), Costume and Fashion as related to film and Children's Clothing and Costume.