Web sites can be useful as stepping stones in your research, even when they're not scholarly sources. Look for lists of publications or a bibliography, to help you find more academic material.
(With thanks to Janine Henri from UCLA Arts Library)
Costume - Metropolitan Museum of Art
Timeline, thematic essays, and objects: all browseable and searchable by geographical region and time period.
Fashion Plate Collection - University of Washington
Plates from some of the leading French, British, American, and other continental fashion journals of the 19th century and early 20th century (from Empire through Edwardian)
The Golden Age of Couture, Paris and London - Victoria & Albert Museum
Interactive timeline of couture houses and couturier biographies.
Textile & Fashion Arts Collection - Art Institute of Chicago
The Museum's Textile and Fashion Arts Collection features more than 27,000 items from American, European, and non-Western cultures and many historical periods.
Textile & Fashion Arts Collection - Boston Museum of Fine Arts
BFA's collection is mainly focused on textiles, but the museum website contains digitized images of fashion illustrations and highlights from their Textile and Fashion Arts Collection.
Casey Fashion Plates - Los Angeles Public Library
Over 6,200 handcolored fashion plates from British and American magazines dating from the 1790s to the 1880s.
VADS (Visual Arts Data Service)
VADS is the image collections database for U.K. higher education. Collections include photographs from the London College of Fashion, the textiles Collection of the University of the Creative Arts at Farnham, and the Howard Resource and Research Centre in Textiles, Goldsmiths, University of London.
FIT's Online Collection - FIT, the Fashion Institute of Technology
More than 100,000 objects; primarily modern and contemporary women's fashion.
Costume and Textiles - Los Angeles County Museum of Art
18th-19th century European costume collection, English fashion drawings by Anne Frankland Lewis, and more.
Kyoto Costume Institute Digital Archive
View costumes by time period from the early 18th century to the mid-20th century.
Historic Costume Collection - Drexel Digital Museum
More than 7000 items including fashion plates, photographs, and 19th- and late-20th-century haute couture by French and American designers.
Digital Library - Wayne State University
Includes the Digital Dress: 200 Years of Urban Style collection. Included are examples of occupational, formal, recreational, mourning, masquerade, and everyday wear for men, women, and children; pieces by European and American designers, local dressmakers and retailers, and home made garments.
Texas Fashion Collection
Digital fashion collection of work by significant designers. (University of North Texas)
Journal des Luxus und der Moden
German fashion magazine, 1787-1827.
Modetijdschrift De Gracieuse
Dutch fashion magazine, 1862-1936
L'Officiel de la Mode
French fashion magazine, 1921-present.
Links page for costume designers. Some pages linked here are very useful, others not so - use with caution and check author information.
Fun for browsing, but no scholarly content
The page titled "what's a good source?" will help you apply these question to your research.