If you need the Form I-94, record of admission, to prove you lawfully entered the US, it can currently be found at: https://i94.cbp.dhs.gov/I94.
In the future, the address of the site may change, but you should be able to find the I-94 admission record on the website of the US Customs and Border Protection agency.
If you have any questions regarding immigration, please feel free to call me, immigration attorney Shawn Mesa at (813) 679-5780.
Disclaimer: This information is provided as a public service and not intended to establish an attorney-client relationship. Any reliance on this information is taken at your own risk.