How to research speech patterns

I have a script idea that takes place around 1906, in a small midwest community, mostly immigrants.

In general though, how do you go about researching the way people speak like that?

The language will be english, but I'm sure the choice of words and phrases will be crucial.

Any thoughts?

First, I would find authors from that time period. Mark Twain (1835- 1910) comes to mind. His writings might help in establishing the way mid-western people spoke. Another idea would be to try a linguist in the research department of your local university to get further recommendations.