Just an add on to what others have said, it is perfectly acceptable to estimate cost basis if you do not have the exact amounts. The IRS can challenge your position, but if it is reasonable, based on other factual information, etc. then that should work. Most likely, this is a computer generated notice and if you file an amendment with the estimate of cost basis, then the issue will be resolved. If you do the best at complying with the law, you will be fine. Don't pay more tax, because you are afraid. Pay based what the true gain was. If you don't know exactly take a conservative approach, but it is better than paying the gain at 100%.
Good luck!