There are a ton of websites out there that will allow you to download fonts for free. I can recommend:
Need help installing a new font? Microsoft offers these steps for Windows computers. Apple has instructions for your Mac here. Chromebook users: Sorry, I don’t see that you can add fonts easily to your devices.
Is there a way to add fonts to Libre? Their list is quite limited.
Yes, LibreOffice pulls from the fonts installed on the operating system. So, if you find, download and install a new font, LibreOffice should show it on the formatting menu, the next time you open and use it.