Not all packages you want are available in Arch repos. In some cases you want to use the Archlinux User Repository (AUR) to install packages. Be careful though, these packages provide no guarantee whatsoever and require manual labor to keep them updated,

Steps

Copy the git URL

$ git clone https://aur.archlinux.org/package.git

Go into the directory and make the package

cd package/
$ makepkg -si

The package will be downloaded, compiled and installed.