I agree with what George suggested.  In addition, if it was always a rental, search the address online.  Zillow and others may show the history of sales of the property and I have been able to obtain information that way.  Also if it was converted to a rental in the past 10 years, some sites show valuations going back that far, that might give you some idea of the value at a specific month.

I have also used the button on the asset entry after guessing what a cost was, to see if the historical depreciation amounts match what was claimed on the return.  In my most recent (current) situation, we are doing a 2016 return and the prior CPA (in a different state) can't be located - yes 2015 was prepared over 4 years ago.  Also the taxpayer only has copies of returns from 2012 forward (spouse probably shredded the older ones), so it was a huge project, but I used all of the above to determine the basis.