You can obtain a copy of your marriage certificate from the local government office tasked with keeping such public records in the state where the marriage took place. This varies from state to state.
The Centers for Disease Control (CDC) website provides specific instructions by state on how you may get a copy. For example, if you were married in California, you would contact the California Department of Public Health - Vital Records. If you were married in Maine, you would get in touch with the Maine Center for Disease Control and Prevention, whereas if you were married in Pennsylvania, you would contact the local County Court House where the marriage license was issued.