I actually got mine off amazon, but it needed inflated, so I had to go to walmart anyways to get a needle for the pump. And right beside the needles were these balls, about $3 cheaper.
Regular sized balls work great for goblets too. They help keep the cup from wobb;ing as the stem gets thin.
Got this idea from Mike Waldt.
Oh and don't use a pointy live center. Use your pen mandrel center, or fashion a rounded cup for the pointy one. Pointy thing + inflated ball = .