On the references for lineage page there should be the authorities for each statement. So if the reference was a birth certificate indicating they were born in Denver Colorado, you could write to Denver for the certificate. You could write and ask the DAR specifically for the backup, but I don't know that they usually provide that.
I know I have a glitch on a line where a very old application referred to a letter. I asked if it was on file and they indicated it was not.
I know it would be nicer to have all this provided, but if you wish to become a member of the DAR, you can refer to the application that has been proven and that is satisfactory.
Hope this helps.