Some fabulous new resources for tracking down extant costume info/pictures online have been added to the Digital Collections of Extant Costume directory:
Europeana — cross-museum search; art from various European museums. I recommend searching both in English and in other European languages. So far, not too impressed by their “narrow by date” options — I recommend instead searching by century, and then again do it in various languages (example: 18th; 18e; settecento; XVIII; etc.).
Esbirky — Czech cross-museum search. Click here to specifically to see the historical costume items in the database. Thanks so much to Hana Marmota for not only finding this, but figuring out how to use it!
Museo San Telmo Museoa in Spain — a few costumes in their collection highlights.
Texmedin — Spanish cross-museum textiles & apparel database. New items still being added, so check back on this one periodically.
Indiana State Museum — thanks very much to The Dreamstress for finding this one!