Marriage records in PA are maintained at the county level. Allegheny County might be the place to search because I believe the attached death certificate is for your Mary based on the name of her father on the certificate.
I have also attached a death certificate for a John, which may be a match.
Some Allegheny County marriage records are online, but, as you indicated, the one you are seeking is not. Here is a link to request a search of Allegheny records. The charge is $4, which will be applied to the cost if located. Since you probably just need a plain copy, the $4 search fee is the same as the plain copy cost.https://dcr.alleghenycounty.us/Marriage/Search/MarriageRecor...http://www.alleghenycounty.us/wo/rec.aspx