Frequently warrants are issued directly to shareholders and if so, the company that issued them will tell you the basis (try their investor relations department).
Other possibilities are that you purchased publicly traded warrants and there would be a record by your brokerage. Sometimes another company whose stock you own has issued them (in that case you need to contact the investor relations for that company).
Broadly speaking, if you don't know what you paid for them, it must not have been extremely large. In that case you can also tell the IRS that your basis is zero. That will not get the maximum deduction possible, but since it wasn't very much the tax on not very much shouldn't be a big problem.