It's your responsibility to have tracked the basis of contributions made to your Roth IRA on your behalf. These would have been reported on Forms 5498 and absent other records that's likely the only place where you would be able to find that information. The IRS will be able to provide you with Wage & Income transcripts showing more recent contributions, but you'll probably not be able to get ones going back more than 10 years.
If the funds are to be used for a first-home purchase, because you've had the Roth IRA more than 5 years up to $10,000 to be used for the purchase can be taken out free of tax and penalty without knowing your contribution basis, although line 22 of Form 8606 is still supposed to show your contribution basis.
