You can find Mary on the FamilySearch or ScotlandsPeople websites (and probably others). On FamilySearch, use
Last name – Martin
Birth Year – 1825 to 1845
Parents, Father’s First Name – John (tick exact)
Parents, Mother’s last name – Leitch (tick exact)
The search should bring up 4 results – John, Ann and Flora McMartin and Mary Martin. Although born Flora McMartin, her marriage certificate shows her (and her parents) surname as Martin. Any ideas as to the reason for the name change?
As for Margaret Leitch, I’ve not seen the 1841 census record for John and Margaret, but I did find an 1851 census record for Archibald and Margaret (she was aged 48 and therefore born @1803). Having looked again at the FamilySearch website, I’ve found Margaret Leitch birth records for both 1801 and 1803 but, while both were born in Glassary, the parent names were Peter and Mary Leitch (no maiden name) and John Leitch and Christian Black respectively.