I've used knak. | Responsive Marketo & Eloqua Email and Landing Page Templates before creating a newsletter in a couple of hours from scratch without html coding. The design would have taken a full week or more to create and qa. Also Pierce Ujjainwalla and his company have the best customer support around.
Etumos also has assets available. I found their landing page Responsive Marketo Landing Page Templates V2 and know they also have email templates too, which you can configure pretty much into any design using the modules. Might just reach out to them and see.