C-SPAN is NOT free. You can only get it from a TV provider. It is not available on any over the air broadcast TV channel. They provide some content via their website, but I can't see a live stream anywhere there. It is only available via cable or satellite providers. Note as I said above that there are some online streaming providers acting as a "cable" company, and as such they offer C-SPAN as available content.
I mentioned previously that it's available for iOS and Android devices. I just checked closer, and see that it's only available for phones and tablets, so an Android based streaming player (like Nvidia Shield or Fire TV) doesn't have an app, just like Roku. So why doesn't C-SPAN offer it on any streaming platform as an app? Only they can answer that question, and only they can make an app happen.