Where can I find more wallpaper packages?

I was wondering if there are any wallpaper packages around

Sure, quite a lot actually. They can be found in your package management like any other package.

wallpapers which are as good as the pre-installed ones

Is a really subjective requirement and there's no definite single answer to your question because of this.

Obvious things to try from here: Use the wallpaper and/or background keywords in...

• finding high-rated listings in the Software Center.
• searching on packages.ubuntu.com or by issuing apt-cache search wallpaper background on the command-line.

Make sure the universe and multiverse repositories are enabled for a wider set of artwork. Feel free to try out any package and remove the ones you don't like.

If you get bored of downloading a lot of wallpapers packages and still in search of some more then Variety will quench your thirst

Installing Variety

• open terminal by CTRL+ALT+T and execute these commands to add repository and install it.

  sudo add-apt-repository ppa:peterlevi/ppa
  sudo apt-get update
  sudo apt-get install variety
  

• Then open variety from Dash

• And get started to dig a lot of features. More you dig more features and wallpaper you'll get.

• I have been using it for a very long time and fully satisfied. Could not find any better than this. Thanks to Peter Levi!

• See more about variety

_{Source: peterlevi.com/variety/}