Quick Answer: RHEL (Redhat Linux) is Open Source. The Source code or "RED HAT LINUX" is available on RedHat Servers, their mirrors as well as scattered about the internet in many respects.
In general, Linux is a bunch of programs and the Linux Kernal. To make a "Version of Linux" or a "Linux Distro" or "Linux Distribution" such as redhat, the "creators" are actually gumshoes that go out and find software that works, they like, the trust, they make scripts and tools to help set things up and they may set up internet sites where you can download what they've cobbled together to make something easy for folks to use.
RedHat does have a commercial offering but I believe it's more about getting/giving support than anything else.
I'm sure with a little bit of hunting on www.redhat.com
you can find where the downloads are...
The Source code is usually there. For some programs, you might need to go to the original authors of specific programs, drivers, whatever... to get the source code and what have you.