Quite a few bloggers have put together incredibly thorough lists of useful resources for both materials and information, and it might seem somewhat redundant of me to add to that. So this is not so much a masterpost as it is a 'these are my favorite places to shop and I hope you'll find them useful too' post.
Sanskriti India: Hands down my favorite ebay store! BEAUTIFUL antique and vintage saris, dupattas, trimmings, etc. Very reasonably priced.
Heritage Trading: Another ebay store with lovely block printed and ikat cotton fabrics, some of which work really well for 18th and early 19th cen.
Decorative Silk: Good selection of decently priced silks (taffeta, duchess, charmeuse, velvets, etc.). Just be sure to order the swatch book(s) since the color in online photes rarely matches reality.
Mellymellow: Spoonflower shop. The designer has not only reproduced several fabrics from Marie Antionette, but also fabric of the wallpaper in the Haunted Mansion. :D
Fabrics Store: All the linen.
Burnley and Trowbridge: Historically appropriate fabrics for 18th and 19th cen. Also where I get linen thread, button moulds, light-weight silk ribbon, etc.
Wm. Booth Draper: Linen, wool, and historical notions. Nice wool tape and satin ribbon.
Renaissance Fabrics: Not the cheapest ever, but I've found some lovely silk there.
Dharma Trading: While this is probably best known as the place to get fabric dye, they do also have some really nice fabrics (cotton lawn I'm looking at you).
Originals by Kay: Wide selection of fabrics that are great for mid 19th cen.
Reproduction Fabrics: Like it says on the tin, reproductions of fabrics from the 18th to mid-20th cen. All cotton. Be careful though... quite a few of the earlier style fabrics are more suitable to furnishings than clothing.
Trims and Notions
Craftemall: My go-to for soutache braiding. They also have beads and other jewelry stuff, but their color selection of soutache braid is outstanding.
The Dressmaker's Shop: They carry fabric as well, but it's the millinery supplies that keep bringing me back. I particularly love their selection of straw braid, which is fairly lightweight and perfect for all sorts of 19th century projects!