If you don’t want to get them from microsoft, you can purchase a license elsewhere. Microsoft allows them to be distributed freely as long as the files are not modified. That’s why they are always packaged in an executable installer.
Those fonts are not free. They may be just ttf files, but there is a massive amount of work that goes into creating a font with unicode support. If you just want fonts for basic compatibility, you can use open source fonts with compatible metrics such as the Liberation fonts or use the microsoft core fonts that haven’t been updated in 20 years.
Basic fonts
What is your “basic” list of fonts every linux desktop user should install ?