Can be downloaded here http://php.net/releases/7_3_0.php
I personally would stick to 7.2 for now at least until the first point releases of 7.3 are out just to be on the safe side.
To install on Ubuntu 18.04:
sudo add-apt-repository ppa:ondrej/php
sudo apt-get update
deb http://ppa.launchpad.net/ondrej/php/ubuntu bionic main
deb-src http://ppa.launchpad.net/ondrej/php/ubuntu bionic main
sudo apt-get install php7.3Xdebug is also available but its currently under beta. Since you won’t be using that in production it doesn’t hurt to try.