I originally posted this in the Marmdog thread in the Freiburg Gazette section, but figured it fit here, too.
While most of my sewing skills are limited to 14th Century stuff (courtesy of the SCA), check out Simplicity's patterns. For women, Pattern 8881 is a good Elizabethan (Avalon noblewoman-ish) pattern, Pattern 4923 is a pirate costume (guess which movie inspired it 
,) and there are some good Ren-fare type costumes for women as well.
Simplicity used to have Revolutionary War costumes as well (think "pirate captain's" frock coats and such) but I don't see them on their website.
Butterick has this, and this one for a Montaigne courtier woman. For a Montaigne man, try this one.
Go into any fabric store and look through the pattern books, specifically the costume sections. I'm sure you can find something that will work.
Also, keep in mind that you can alter patterns to fit what you need. By using fancier material, you get nobility, but by using plain material, or only parts of the costume, you can get a lower class costume.
For those in Michigan, you just missed Val Day at the Kalamazoo County Fairgrounds (SCA event, so it's more medieval, but they have patterns there for some things, and silk for $5 a yard) but on March 19-20, there's a Living History show there that has all sorts of stuff for re-enactors, including patterns for 18th Century clothes, and all the accessories you could possibly need. It's also really neat in and of itself.
Hope this gives some people some help.
Edited to add:
Keep an eye on Jo-Ann Fabrics and Hancock Fabrics, especially around any holidays (like the upcoming President's Day.) They usually have sales and mark their patterns down to .99 each.
