You can view the source code for a sent mailing via its Mailing Activity Report.
NOTE: Because you're looking at the mailing in its entirety, the source also includes the HTML from the template.
- Navigate to Mailings > View.
- Click on the Sent tab.
- Click on the mailing name.
- Go to the Published Mailing tab.
- Click the View Web Version link in upper right to launch the browser and display this mailing
- Place your mouse cursor to the edge of the screen and right click.
- Select View Source Code or View Page Source.
- Select the HTML that you'd like to copy. Use Ctrl+C to copy it and Ctrl+V to paste it.