Suppose I have one of a pair of sending stones, and I wish to locate the other stone of the pair. Is there any spell or other magic that will allow me to find the stone's mate? Assume I have never held or seen the stone's mate.
A limited range solution is potentially acceptable, but a longer range or unlimited range solution is preferred (even more so if it works across planes), since I have no way of knowing where the other stone is. However, any solution must locate the specific stone, not just any sending stone, not least because unless I have a bag of holding, the nearest sending stone is always going to be the one in my pocket. If there are any additional limitations to the solution, be sure to note them (e.g. "the stone must not be in the possession of a creature", "the stone must not be inside a lead box", etc.). Lastly, casting Wish and wishing to know the stone's location should obviously work, so there's no need to mention this option. If you like, consider Wish the "baseline" solution: your solution should be less demanding than one that requires a 9th level spell slot, multiple days of rest, and risking losing access to Wish. In short: the best solution would have the longest possible range and the fewest possible limitations.
One additional note: since a lone sending stone wouldn't be of much interest unless it was in someone's possession, you may assume that the stone is held by a creature, and a solution that locates the creature in possession of the stone is also acceptable, since this would also locate the stone. However, assume I have never met this creature and know nothing about them, other than that they possess the stone.