Amazon pulled the Chromecast from its online store in 2015, after it instated a new rule that only streaming devices that supported Prime Video could be sold. Of course, Amazon itself dictates if Prime Video is available for a given platform, so many saw that as a thinly-veiled attempt to boost sales of Fire TV products. Three years after they were pulled, and a year after Amazon said they would re-appear, Chromecasts are finally back on Amazon.com.
Amazon announced that Chromecasts would return to its online store last year, seemingly as a good-will gesture towards Google. At the time, the two companies were fighting a petty battle over YouTube on the Echo Show. Product listings for the Chromecast and Chromecast Ultra went live at that time, but they simply said "Currently unavailable" — until now.
You can now purchase the third-generation Chromecast and the Chromecast Ultra from Amazon, for the standard prices of $35 and $69, respectively. They're shipped and sold by Amazon itself, and Prime shipping is available.