Update: following Steam's announcement regarding the future of the Steam Link, units are currently on sale for $2.50 (though expect to pay shipping, of course).
We said in May that the Steam Link app for mobile devices, which enables wireless streaming from PC to television for couch-comfy gaming, works pretty well but "doesn't quite make the physical Steam Link box obsolete." Nonetheless, obsolescence appears to be here, as Valve warned today that Steam Link hardware supplies are almost completely depleted.
"The supply of physical Steam Link hardware devices is sold out in Europe and almost sold out in the US," it said. "Moving forward, Valve intends to continue supporting the existing Steam Link hardware as well as distribution of the software versions of Steam Link, available for many leading smart phones, tablets and televisions."
The message doesn't explicitly state that Steam Link boxes are discontinued, and as far as I know Valve hasn't publicly stated so either, but it seems clear enough that it's shifting its efforts to the app. And understandable, really: It's a lot easier and cheaper to update an app than to maintain an internationally-available supply of boxed hardware.
It's not known how much inventory remains available, but if you want one you should probably put your order in right now: As of this moment it's still listed as available in the US, but in Canada—which is right next door—it's already showing as sold out.