How about reading a contemporary novel, published during the time you are interested in? Then again, remember that regional dialects were more distinctly different in the age before mass media.
In talks about this subject with my writers' group, we've come to the consensus that you should (mostly) write it in a basic style that is timeless, with just a hint of the speech patterns of the time. After all, if you tried to translate Arabic (have you ever heard a translator's "word for word" translation of a speech? Sentence stucture is different in Arabic, and it's virtually incomprehensible.) it would come out as gibberish unless you "Anglicized" it. Same with historic speech, it needs a bit of polishing. I'm currently reading (via audio book) William Faulkner's "The Light in August" written in 1931 with period southern dialogue and narrative. WAY overdone and hard to follow. Not for the people who read it in the '30's perhaps, but hard for me.
The big no-no is using modern terms that completely jar the reader out of the period, like an 1800's character saying "off the hook' or describing something as faster than the speed of light, when they didn't know light had a speed back then.
Just my thoughts. Hope that helps, because I don't think there's a source for historical speech patterns.
Take care,
JohnB
Did you have a particular era and language in mind?