If you don’t know about Project Gutenberg, you are in for a treat. It’s a collection of free e-books, in a variety of formats, readable on e-readers and the computer. Most are non-fiction but, luckily for fantasy readers, that includes fairy tales and folklore!
The 1811 Dictionary of the Vulgar Tongue would be a great resource for linguists, word enthusiasts, and writers setting something in that era. It’s also uniquely readable; you can scan it in a way that you can’t with a modern, serious dictionary. However, I have to note: I don’t think even the writer could really ascertain any authenticity here. Maybe since communication was slower in that century linguistic fashion was more stagnant, but I imagine many of the entries would have sounded to contemporary ears like our grandparents trying to be “hip.”
My favorite contribution is a synonym for “topsy-turvy”– ARSEY YARSY. It will definitely be entering my vocabulary.