When apt-get install is unable to locate a package, the package you want to install couldn't be found within repositories that you have added (those in in /etc/apt/sources.list and under /etc/apt/sources.list.d/).
The following (general) procedure helps to solve this:
1. Make sure you have enabled Ubuntu repositories:
To enable all repositories (main, universe, restricted, multiverse), use the following commands:
sudo add-apt-repository main
sudo add-apt-repository universe
sudo add-apt-repository restricted
sudo add-apt-repository multiverse
2.Add PPA (by command-line):
Use this command:
sudo add-apt-repository ppa:<repository-name>
3.Don't forget to update (make apt aware of your changes):
It is essential to run this command after changing any repositories:
sudo apt-get update
4.Finally install the package:
sudo apt-get install <package>
Note: If package is not available on repository any how, than you have to wait until it is available (in the case of new/updated versions) or use other installation processes than apt-get
e.g. compiling from source, downloading executable binary, etc.