CR2032 coin batteries
You can get 10 packs of these CR2032, name brand like Sony or Maxell, for around $1.50 each from online battery stores. Larger quantities are close to $1.00 each.
There's lots of no-name batteries on Amazon third party sellers for much less than $1.00. I wanted reliable ones for my monitor devices, but for a spoke light, the really cheap ones would be good.