I had a look around to get an idea and looks like the original was a chromed plastic target, so something like what Zitt would work and then found a mayfair listing on worthpoint and there is a used set on ebay.
https://www.worthpoint.com/worthopedia/tommy-pinball-replacement-silver-ball-411384217
photos are of the used set on ebay
also one could be designed but would need one to replicate from