...can not find the original marriage certificate...
If for any reason, you do not have your parent's original marriage certificate, then your parents would have to submit a sworn affidavit stating that they are married, when and where they were married along with their full names and date of birth. This affdavit needs to be sworn before an administrator of oath (or an equivalent person authorised to administer such an oath). If there are any other documents that has both your parents names on it, provide copies of that too. in your cover letter, mention why you were not able to provide the original marriage certificate and what documents you are submitting in its place.