Per the instructions mailed to your home, you have until the deadline to submit documentation. To help you, information on resources is in the letter mailed to your home, and there is also a website set-up for this process at www.dsiverify.com. There is a resources page with links to help you find any public record in the US, like birth certificates and marriage licenses.
You may also consider calling the county clerk where the birth or marriage took place.
They might charge you a fee, but it is a viable alternative to the online process. Just don’t delay in starting the process because it can take 2-3 weeks to get documentation and the deadline is soon approaching.