There are 500,000+ websites related to sewing historic garb. It’s overwhelming. A good place to start if you have all afternoon is (my favorite) The Costumers Manifesto. She has an index listing time periods, each page containing many links to useful resources for that era.
Jennifer, of Cloak & Corset, has written what looks to be a comprehensive and invaluable tool for those interested in sewing history-based garments Modern Sewing Techniques for Historical Clothing Construction.
Places to buy historic fabric, patterns and notions:
Acme Notions A large variety of vintage, modern and unique sewing notions.
Burnley and Trowbridge Company have been hired by museums and historic sites around the world to create historically accurate garments. They have a track record of excellent work. They do classes, based in Williamsburg, VA. You can buy fabric, patterns, notions and tools here as well.
Elizabeth Stewart Clark, a long time re-enactor, has a useful site, including this free pattern page. She specializes in the “Crinoline Era” (1840s-60s).
Timely Tresses covers historic millinery from the Regency through Civil War Eras. You can buy original patterns and kits based on actual historic headwear. Many antique fashion plates also available for research.