The maintainer hasn't built the .deb packages for 13.04 yet. And I don't think you'll find them anywhere else.
You could wait who knows how long till he gets around building them, or downgrade to 12.10. Or you could do what I and many others do: Compile it from source yourself. It's pretty straightforward, and this approach comes with many advantages such as giving us the ability to make optional customizations, such as adding support for more codecs for example.
FFmpeg provides a step-by-step tutorial, which I myself have followed many times without incidents so far:
http://ffmpeg.org/trac/ffmpeg/wiki/UbuntuCompilationGuide
You can uninstall your custom build and go back to the repositories versions any time you want(but honestly why would you).