Google upset quite a few people when it discontinued the Chromecast Audio earlier this year, but there are plenty of alternatives. Amazon's Echo Input turns any speaker with a 3.5mm audio input into a Wi-Fi enabled speaker, and now you can get it for just $9.99, a drop of $5 from the already-low Black Friday price.
The Echo Input is actually more useful than the Chromecast Audio was, as the included microphone allows your speaker to function just like a full-blown Echo smart speaker with the Alexa voice assistant. It can also pair with Bluetooth-enabled speakers, if a wired 3.5mm connection isn't possible for some reason.
You can grab the Echo Input from the link below. Both the black and white colors are available, at least as of the time of writing.