When dealing with evolutions involving the use of specific items, some will only be used once depending on how general their applications are. The exception to that rule is typically Evolution Stones or other items dealing with typings because they have a wide range of Pokémon they can apply to.
Pokémon Legends: Arceus uses all of the Evolution Stones at least once, with some, like the Thunder Stone, being usable on multiple species that appear in the game.
The item itself can be found in a few different ways, and you will need at least one if you want to evolve your Eevee into Jolteon. It can also be used on Magneton if you don’t want to wait until the late game to evolve it into Magnezone.
The Thunder Stone, along with most of the original Evolution Stones, is a reward option for finishing Flaro’s Eevee’s Evolution Request.
You can actually purchase a Thunder Stone at Ginter’s Specials in Jubilife Village for 5,000 PokéDollars. Or if you would rather use the Merit Points you have saved up from returning lost satchels, you can head over to the Trading Post and grab one for 1,200 Merit Points.
Thunder Stones can spawn in space-time distortions along with the other Evolution Stones and some various rare items. And while exploring the map, you can break grey rocks in various areas for a chance to find one in the rubble.