GC: Previous Employer Denies experience letter

Hi All,
I am applying for GC and my previous employer (first employer in my case) is not ready to provide Experience Letter as it is not their policy to issue experience letter, they confirmed over email the same.

I’ve heard about getting experience letter affidavit notorized by co-worker from ex employer as a possible alternative. Can this be a co-worker or it MUST be my ex-supervisor?
What if the co-worker is no longer with my ex-employer anymore?

Please suggest.

Thank You.

It is recommended to get letter from supervisor.
If not possible, then co-worker should also work.

Get it notarized and it should be okay even if the person is not working with the employer anymore.

thanks for the reply @anil_am22.