This has been a very problematic situation for many clients. Here's what I suggest, estimate the price of the stock you might have paid 20 years ago!
I know it is extremely hard to do so, but use your best judgement or go to the library and get a Stock Market pricing history data book or some reference book that has Stock Prices data and look up the price of the individual security 20 years ago! You might be lucky and find some pricing data, and come up with the average price of the stock 20 years ago.
Once again this is an estimated, but at least you can document how you arrived at the price and the IRS will generally accept this method of pricing.