Father's marriage certificate not available

[mehtaabhi]

i am a US Citizen and want to apply for green card for my Dad. My mother passed away a few years back and my father does not have a marriage certificate. He tried getting it made in India but was told it is not possible since my mom would have had to sign on the form for getting the certificate.

Is there a way out? I read that I am required to submit my parents marriage certificate for sponsoring my Dad for green card.

Thanks
Ashish