The same has happened to me few times and sadly I've never been able to find a good replacement.
I saw an eBay listing for a pool/billiard ball claiming to be compatible, but that's not apealling to me, and the price was something ridiculous like 40-50€, so I learned to live with the pain of just rotating the ball upside-down when I hit the dent.
I guess it would be possible to do a tiny patch on the dent with epoxy, but I don't want to take the risk of making things even worse.