To clarify, are you are taking a Roth distribution this year and you made conversions prior to 2017 but you don't have any records (tax returns, old bank records)?
Are you under 59 1/2? If you are over 59 1/2 then you do not have to worry about this information because your distribution would be a Qualified Distributions and therefor not taxable. The information is only important if you are under 59 1/2 to calculate your basis in the Roth IRA since only earnings will be taxable when it is an early distribution.
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