If 1099-B for sales of stock shares purchased from 1996 - 1999 has no cost basis in box 1e, how do I report the cost basis?

I still can’t find it. 

You should contact the seller or find your own record if the financial institution who issued you the Form 1099-B didn't provide you the information. If you won't find what did you pay for the stocks, entire amount proceed will be taxed as a capital gain.