samtto We've had many people who don't understand our use of Composer, and would prefer a direct download. We understand Composer can seem overwhelming, but it's a simple tool at its core. We highly recommend you make use of this method.
If you really feel confident about your skills, you can find the full source code at flarum/flarum and flarum/core. You will need to compile the JS yourself via NPM Webpack (you can find resources on how to do this online). As clarkwinkelmann said above, we do not recommend IIS, nor do we recommend installing without Composer. However, if this is within your capabilities, feel free.
We will unlikely to be able to assist if you get stuck in this case. We highly recommend you use a compatible webserver (Apache, Nginx or Caddy) and Composer to install Flarum, we do provide support for these methods.