Below is a list of sites that I've used to source fabrics for my various projects. I am not affiliated with any of these companies.
Burnley & Trowbridge (B&T)
Best source for reasonably priced wool
Up to 4 free swatches if you contact them
Period accurate to 1700s
Provides thorough info on fabric weaves
More variety and color selection than B&T, less historically accurate, comparable pricing
Slightly more of a focus on medieval and renaissance fabrics over 17th-19th century
Up to 4 free swatches
Lots of color, weight, and weave variety (some of the finest quality wool I've ever gotten my hands on)
Comparable prices but shipping costs are high
Free swatches, even all the way from Germany!
Wools in a great variety of weights and colors (I highly recommend the German wools)
They also have linen and silk, although I have not tired them
Comparable prices but shipping costs are a factor
A nice selection of Colonial era wools, linens, and cottons
Similar to B&T but sometimes has different stock
Fabric-Store (FS)
Linen in lots of weights and colors
Good quality (great for beginners and budget sewers!) but short fibers make this linen less good for techniques like thread pulling
Best price for white linen by the bolt (20 yds)
Mid-weight and handkerchief weight linen in lots of colors
Located in NYC if you want to go to a brick-and-mortar store
Slightly more expensive than FS, but slightly higher quality fibers
One of a few places that carries linen twill
Sort of expensive
HUGE variety of silk colors and a solid selection of weaves
New colors regularly
Reasonable prices (for 100% silk)
Lots of kinds of fabrics, but most special are their historical silk brocades and velvets
Expensive both for fabric and shipping (this is your fanciest garb fabric)
Only source I've found for silk twill in colors (Thanks Lady Marguerite de Lyon!)
Get the 19mm "deep twill" for a closer-to-historical fabric weight
Has a lot of other silk options as well for fairly reasonable prices
Best source for undyed silk in many weaves
Great source for dyes
End of bolt one off fabrics, low prices, varying stock
Cheap muslin for patterning
Miscellaneous discount fabrics, sometimes good wools or linens
Source for ramie (nettle linen)