So, I'm pretty stoked that we've now got a historical costuming forum! To celebrate, I'm starting a Compendium of Knowledge for online historical costuming resources. This is by no means a comprehensive list, nearly all of my experience is with 19thc women's clothes. Feel free to add your own links.
[b]Shopping: commercial patterns[b]
Truly Victorian: http://www.trulyvictorian.com
Probably the best commercially available Victorian patterns for women. Recommended for beginners, since all patterns include detailed fitting instructions.
Laughing Moon Mercantile: http://www.lafmoon.com
Men's and women's patterns from the mid 19th c up through the Edwardian era.
Past Patterns: http://www.pastpatterns.com
Men's and women's patterns from the 1780s through the 1940s. They also sell re-prints of original period patterns.
J.P. Ryan: http://www.jpryan.com
Men's and women's patterns from the Georgian era. Expensive, but I hear they are worth it.
Sense & sensibility: http://www.sensibility.com
Women's and girl's patterns from the Regency, Romantic, Edwardian, 1910s and 1940s. Best known for the Regency patterns, and highly reccommended for beginners because of the detailed instructions and some photo tutorials on the website. Website also includes a database of extant dresses and fashion plates.
Reconstructing History: http://www.reconstructinghistory.com
Men's and women's patterns from the Medieval era to the 20th century. Website also has many useful article, and they're expanding their inventory to include period correct materials and supplies.
[/u]Ageless Patterns:[/u] http://www.agelesspatterns.com
Re-prints of original period patterns. Proceed with caution, these have minimal instructions, come in a single size, and are not for the faint of heart.
Lynn McMasters: http://www.lynnmcmasters.com
Hat patterns from many eras. The website has useful hatmaking articles and Lynn's beautiful portfolio.
Mantua Maker http://www.mantua-maker.com
Women's patterns from the Renaissance to the Victorian era.
Shopping: Supplies, materials, and accessories
Timely Tresses: http://www.timelytresses.com
Bonnets and supplies for the 19th century.
Reproduction Fabric: http://www.reproductionfabric.com
Cotton fabrics in reproductions of prints from the 18th through 20th centuries.
(LA store) http://www.farthingales.on.ca
(Canadian store) Corset making supplies and more
Vena Cava: http://www.venacavadesign.co.uk
UK based corset supplies and more
Judith M. Millinery: http://www.judithm.com
Stuffs for making hats
Reproduction shoes, stockings, and buttons from the 18thc and Civil War
Jas Townsend & Son http://www.jas-townsend.com
Pretty much anything you could want for the 18thc.
Free patterns and how tos:
Elizabeth Stewart Clark: http://www.elizabethstewartclark.com/gamc/index.htm
Mid 19th c tips and tricks, and a handful of patterns.
La Courtouiere Parisienne: http://www.marquis.de
Many free patterns and tutorials from many eras. Mostly for women, but there are a few men's things hidden in there too.
Demode database of real women's clothing: http://demodecouture.com/realvict/
Links to photos of extant clothing from 1600-1919
Koshka the Cat: http://www.koshka-the-cat.com
Mostly a portfolio site, but the webmistress also shares detailed photos of antique clothing, fashion plates, and photos.
Many, many museums:
Too many to list. A lot of big museums have photos of their collections online. Think of a well known museum, google it, and see if they have anything relevant to your interests on their website.
Yes, eBay. You can usually find fashion plates, antique clothing, etc on eBay, and save the images to your computer for free research. However, proceed with caution, the seller could mis-represent the product or just have no idea what they're talking about. Still, fashion plates are pretty safe.