The only ones we currently have published are at http://templates.marketo.com
We're going to be adding a ton soon. So stay tuned. In the meantime, there are tons of resources across the web where you can download free templates.
We had to just redo ours due to branding updates. We used an outside vendor to create a responsive code and then we updated it with the Marketo <div> tags.
Hoosh Marketing has an Email Template Builder that allows you to quickly create customized emails, with no coding required. You can create and edit the template in the tool and paste the html directly into Marketo.