You are not likely to find a 1942 marriage certificate online, due to privacy issues. I would suggest you try to pinpoint where the marriage took place. Then contact that place (the town or county) to find out on what level (town, county, state) the records are kept and then acquire a copy that way. If they were likely to have married in church, churches also keep records, here again you will need to know where and what denomination, or you're searching for a needle in a haystack