I generally buy my Rokus from Amazon, but if they are on sale at Amazon, they are probably on sale at BestBuy and other electronic stores. I should also note that Roku TVs (smart TVs that use the Roku interface as their "smart" GUI) also work. I have a couple of "Roku TVs" that work great with the Spectrum app. As you noted above, Samsung Smart TVs also support the Spectrum app. I actually have one and can confirm it works just fine.
I always get excited that something has changed when I see something that shows Chromecast as being supported by the SpectrumTV app. However this is not really the case and "support" simply means you can "cast" SpectrumTV to the Chromecast from another device that actually supports SpectrumTV. There isn't a Chromecast Spectrum app like their is on Roku. (I have a Chromecast and can confirm that there is definitely not an app available).
Basically there are three workable options for easily watching SpectrumTV on your television set - using a Samsung smart TV and downloading the app from the Samsung store, getting a Roku TV/ Roku streaming stick and connecting it to the TV and downloading the app from the Roku store, or using the Spectrum app on the XBox One. Using the XBox One doesn't seem like it would be a seamless TV watching experience, but I have never tried it myself. Everything else is a "hack" IMHO and not convenient to use.