Snorunt can learn Weather Ball from the following Pokémon who learn it through level up, depending on the game,
Castform - HGSS, Gen 5 - Gen. 7, and BDSP
Roserade - HGSS, Gen 5 to Gen 7, and BDSP
Vanilluxe - Gen 5 to Gen 7
I'd generally recommend Roserade, as going to the Move Reminder with it takes less time than levelling up the other options. Otherwise, Castform learns it in all games before Vanilluxe evolves.
In Sword/Shield and Scarlet/Violet, Weather Ball is no longer an egg move, so Snorunt now learns it through TM. Snorunt can also learn Weather Ball in SV at level 50.
