Unfortunately, modern glasses and historic costumes don't blend very well. I remember when I was 14-15 and refused to wear my glasses when in costume (my mother thought I was too young to have contact lenses): I didn't see a thing, but I thought it was better than sporting my octagonal brass-framed glasses (yes, they were very weird. And besides, at that time my costumes sucked so much that no glasses could possibly have spoiled anything).
Anyhow; I've seen many people use early 20th century spectacles of this kind, which looks pretty awesome in my opinion. The good thing is that you can usually find them at reasonable prices in antique/vintage clothing stores (myself, I use contacts when I'm dressed up - it feels very glamorous since I'm too lazy to wear anything but ordinary glasses to work and when at home these days)
An 18th century example from Antique Spectacles:
...and some examples from 18th century art:
Goya; Self-portrait with spectacles
Two self-portraits by Chardin