Those snooker balls are actually 2 1/16", 2mm too small.
It's very hard finding 55mm (2 1/6") balls, I've been through this before. Where balls would need to be made to precision, like snooker/billiard balls, they don't do them in 55mm sizes. You might be able to find 55mm baoding balls, but there's no guarantee on their size, they could be 53-57mm, I'd say it would be the same story with juggling balls.
I tried my luck on two rocks from ebay. One was easily too large, the other, a very nice black obsidian ball with silvery/gold sparkly bits, I believe is 55mm but not perfectly spherical, it's usable but occasionally seems to want to jam, it's also very scratchy due to a plethora of small surface imperfections.
I'd get onto Kensington again, that they won't provide replacement balls sounds like a crock of ****.
Good luck.