Background verification - employer asking for experience certificate

sadathu
Ive switched two companies during a 9 year tenure. I spent 1 year with the first organization , 5 years with the second and 3.5 with the third. My 3rd or current employer has ask me to provide an extra experience letter for my first employment (that includes joining and relieving dates).
I only received an offer letter, a confirmation letter and a relieving letter from my 1st employer. Since its been a long time, 6-7 years I'm unable to locate them as they have shut down the process i worked in. They have stopped responding to my emails and my current employer keeps pressurising to get the document despite the fact that my documents are valid. What are my options? Who should i escalate this matter to?
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