Custom fonts on landing pages is easy. If you're using something like Typekit (which I would recommend), you can plug in the JS code it generates into the landing page directly in a custom HTML block. Better yet, you can incorporate it into the landing page's template.
Email is a bit trickier.
Webfonts aren't universally supported by email clients. That said, you can try using the
CSS @import rule to yank in a Google font you like. That call would have to be in the template for your emails. Moreover you'll need to ensure that you reference appropriate backups so the design gracefully degrades in case it won't render your custom font.
.png)