The best way to farm the Sun stone is at PokéStops. Although you can also do it by completing the special investigations, the investigation tasks timed in the weekly events and the weekly field investigation tasks for seven days in a row. This last form will give you a chance to catch a Monthly Advance Research Pokémon and various items, hopefully including the Sun stone.
But you must bear in mind that not all weekly or special investigations contain these evolution elements, which are usually given as rewards if a Pokémon has its shiny version, or it will be one of the augmented spawns that you can capture throughout the week. That's all we can tell you about how to get a sun stone in Pokémon Go, so now you have it, we hope you can get the most out of this item by evolving your Pokémon.