New, extra fonts you can download from many different sites, and here are some of them:
installing and changing new themes in Ubuntu.
CLI wayOpen terminal and:
cd /to/folder/where/downloaded/font/archive/is sudo mv FONT_FOLDER /usr/share/fonts/
I prefere to put .ttf fonts in /usr/share/fonts/TTF folder and .otf fonts in /usr/share/fonts/OTF folder, but it's not a rule. It is important, only, to put desired font files in /usr/share/fonts folder.
After that, no matter whether you did it through GUI or CLI, you'll need to refresh your new font cache if you want to use new fonts, and you'll want it. Well:
sudo fc-cache -f -v
New fonts are now available and you can use them in your work. To change default fonts in your system, open Unity Tweak Tool, search for Fonts section and set your favourite fonts.