As SweetieJean mentioned, your first source would be your Roth IRA account statements. However, the Roth IRA custodians must report to the IRS on Forms 5498 the contributions that you make for each year, so these for each year are what the IRS goes by. If cannot locate your copies of these Forms 5498, you can find these reported in your Wage and Income transcripts for each year beginning with 1998, the first year for with Roth IRA contributions were permitted. Your most recent Wage and Income transcripts are available online: https://www.irs.gov/individuals/get-transcript
For older Wage and Income transcripts you'll need to submit Forms 4506-T: https://www.irs.gov/pub/irs-pdf/f4506t.pdf
Note that your eligibility to make a regular Roth IRA contribution is limited by modified AGI, so for any years that your modified AGI was above the limit you should not have made a regular Roth IRA contribution.
You can also receive basis in Roth IRA contributions from the rollover to an IRA of a Designated Roth Account in a qualified retirement plan such as a 401(k). The basis available to be rolled over would have appeared in box 5 of the Form 1099-R reporting the distribution from the Designated Roth Account. Forms 1099-R are available in your Wage and Income transcript for the year of such distribution.
Also note that your basis in Roth IRA contributions will have been reduced by any prior distributions you have made from your Roth IRAs.
