This happens when the package list is not up to date.
1) First you have to update package archive list by running following command;
sudo apt-get update
2) Once package archive list updated, run following command;
sudo apt-get install python-wxgtk2.8
Further reading @ Ubuntu Package Management from Command line using apt
** However, if you have mistakenly removed/disabled “apt” repositories, it would be easier to use GUI based repository management tool to reconfigure “Software sources”.
Have a look @ GUI-based repository management