Stacie,
You've been given good advice already. I too suggest Amazon Dry Goods (
http://amazondrygoods.com/) whose catalogs of vintage patterns, reproduction shoes & hats, etc are superb. And eBay (
http://www.ebay.com/) has a trove of good things - search under "Edwardian" (though don't be fooled; some dealers haven't the foggiest what that means!)or "1912 dress" or any other year you're looking for, etc.
Shell has suggested Douglas Gorsaline's "The Way We Wore" which is certainly a good starting place for familiarizing yourself with the general line and feel of clothes (men's as well as women's) of the era. I would recommend in addition Lucy Barton's classic "Historic Costume for the Stage." It gives great overviews of every period of dress from the Egyptian to the beginning of WWI, complete with detailed line drawings of all manner of styles for men, women, and children. There is also practical advice on how to reconstruct costumes.
This book has been through many printings over the years though I believe is now out of print. Still, try some of the online book search stores - bibliofind.com & bookfinder.com, etc - and you should be able to find it reasonably priced. Also any large library should have it.
One more book may help - Norah Waugh's "The Cut of Women's Clothes," first published in the 60s but is now out again - contains scale patterns for costumes from the 17th century till the 1930s (I think), including patterns for designer dresses now in museums, such as one for a 1911-12 gown by Lucile,
Lady Duff Gordon in the Museum of London.
Randy