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Where did you get the plastic ball?
Jim got his in Australia ..... which may not work too well for you I'm thinking!
I suggest you try your local hardware store first, as some do stock such handles and knobs.
If that doesn't work, I'd recommend one of the machinist sources like MSC, Grainger or similar.
If all else fails I might know where there are a few hundred 1" balls, but I'd guess you only need one! ;c)