I need Courier to my fonts. Courier New is included in the fonts folder, but I need regular Courier.
Buy the font. Install it using your normal operating system procedure. Restart LibreOffice so it will read the Fonts folder again.
As @EarnestAl said, installing fonts is a OS thing, so you need to get the actual font and install it using tools in your system. But instead of buying a proprietary version of Courier, I recommend you to consider opensource alternatives such as Courier Prime.