1099-B, box 5, how to figure out cost basis?
For every document that the IRS posts, there will be a correlating instructions page. This would be the instructions for the 1099-B, here. Furthermore, as you will be reporting this on Form 8949, as a substitute for previously used Schedule D; instructions are here.This article explains that the best course of action is to donate the shares as the cost basis would switch to FMV (fair market value) of the assets today. But as this did not happen, I would recommend contacting the purchasing company directly. Being a share holder, and by purchasing the shares from the source, the accounting department should still have recorded the date of purchase along with the price sold. It may take effort to prove who you are, but if their accounting records are well documented, this will not be an issue.  If nothing else, claim a 100% capital gain on the entirety of the sale, and pay the tax. That is stated here.