I paid late self-employed taxes and need to mail the physical receipt to IRS, but I can't find the document I need to send them. Where can i find it on this site?

@kris-jeffers 

Please explain exactly what you are trying to find?    What exactly is the IRS asking for?   Proof of paying taxes would be your bank statement, cancelled check, credit/debit card statement, etc.

TurboTax free Audit Support Center can help you with how to respond to an IRS letter/demand.

Click the blue "Get Help" button at this page and follow the prompts to get the phone number:

https://support.turbotax.intuit.com/irs-notice/audit-support/

How did you pay the federal taxes owed?

Did you use the IRS website for tax payments - https://www.irs.gov/payments