What did you sell? stock? bond? mutual fund? If it's stock, and you know approximately when you bought it, you can look up the market price at the time and use that as a reasonable estimate of the basis. There are several web sites where you can look up historical stock prices. For stock that you bought that long ago, an approximate purchase price will be good enough, as long as you have a reasonable basis for the estimate.
Here are a few sites you can try.
http://bigcharts.marketwatch.com/historical/
http://www.marketwatch.com/tools/quotes/historical.asp
https://finance.yahoo.com/quote/MSFT/history?ltr=1
What did you sell? stock? bond? mutual fund? If it's stock, and you know approximately when you bought it, you can look up the market price at the time and use that as a reasonable estimate of the basis. There are several web sites where you can look up historical stock prices. For stock that you bought that long ago, an approximate purchase price will be good enough, as long as you have a reasonable basis for the estimate.
Here are a few sites you can try.
http://bigcharts.marketwatch.com/historical/
http://www.marketwatch.com/tools/quotes/historical.asp
https://finance.yahoo.com/quote/MSFT/history?ltr=1