There are several great resources for tracking historical tornado information across the U.S. Of course, for Illinois I have a web page devoted to tornadoes here. The two major US sources are the NOAA National Climatic Data Center and the NOAA Storm Prediction Center. The first site is great for generating reports while the second site has a wealth of maps and plots. In fact, the second site has so much material that I will address that one in a separate post.
National Climatic Data Center
The URL for their database is https://www.ncdc.noaa.gov/stormevents/ and it allows you to select the state, a range of dates, type of events (tornadoes, hail, etc.) and which counties you want. It will come back with a list of reports, which you can then click on each one to get the full report. It is great for documenting severe weather. Below is an example of the kind of report you can expect.
My suggestion is that you start your search over a small geographic area and a short time span to get quicker results. Here is an example of the kind of detail in their reports. The reports span the period of January 1996 to December 2013 and is updated periodically.