Experience Certificate

I started working in 2006 with a very small firm. I switched job in 2009. I have experience certificates of all the firms from 2009 onwards but do not have it for my first job. The company was so small that it’s difficult to track down anyone who is still there, not even HR & hence I don’t see any way of getting the certificate from that firm.
What should I do in this case?

Submit paystubs, W-2s, anything else officia, that you can find w.r.t. that company along with LoE.

Can you guide me how to provide work evidence, if I had been paid on cash?