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they definitely seem like something that's only about in the US.
They're available on the UK versions of Amazon and Ebay. It seems like they usually have a second ball inside them, in both the US and the UK. I don't know whether Mel was able to find one that was empty, got the smaller ball out somehow, or just simply covered it up.
The exercise balls for hamsters have potential too, but they're usually see-through not opaque.