Go to fonts.google.com then search for Thai font family, select the one you like or many as you want. Click Download, you will got a zip file contains that font.
I don't know about other distro, but don't use aur/ttf-google-thai because it doesn't contain any glyph of Thai font.
You can also use noto-fonts but it is 108MiB large, to large if you only want thai font.