How many buildings in tornado prone-areas have storm shelters?
Have you perhaps contact the National Weather Service or FEMA about that ? If not, then maybe find out from real estate that they might have data for each houses in your state. They may have the data or not, but it is hard to know. All I know is that each state has different types of soils that can or can't built storm shelter. I heard in Texas almost 1/2 of the buildings there do not have a storm shelter, because the soil in Texas are not strong enough to withstand against the flooding. That kind of thing ...

I do not know if you have seen this, https://stateimpact.npr.org/oklahoma/2013/09/27/mapped-which-oklahoma-schools-have-storm-shelters/

http://www.esri.com/news/arcuser/0104/storm.html

Another one is the reason why an individual passed away from flooding in a storm shelter
http://mashable.com/2015/05/07/oklahoma-city-storm-fatality/#KGEHNAxU5sqq