What do I use for a cost basis if my 1099-B does not even have a box for cost basis? The stock was acquired over 20 years ago and is only worth about $1500.

You are going to have to do some legwork-  see How do I find a stock's cost basis? and How do I find a stock's cost basis if I don't know when it was purchased? 

Be sure to keep records and notes of what amounts you use and if not sure, be reasonable.  If you cannot find the basis, you may have to use a zero basis and pay tax on the total proceeds.