They’re relatively standard, if you take a PLAY:1 with you to your local hardware store, you should be able to get replacements relatively easily. I seem to recall the they were ¼ inch 20 thread count, around ½ inch in length, but take that with a grain of salt, and merely as a starting place. 

Hi Bruce,

Many thanks for advice, much appreciated.

Tim

If you look carefully, you’ll notice that it’s the same thread as a camera mount.

Thanks  for info

Appreciated 

They aren’t normally provided by Sonos, except if you purchase a stand, definitely not as part of just a speaker. 
 

As @buzz suggests, this is indeed kind of a standard screw type. When I wanted to shop for them, the hardest part was figuring out how long it needed to be to fit the ‘mount’. As I recall, I ended up adding a few spacers.