Getting a Cetoddle to evolve into Cetitan in Pokemon Scarlet and Violet isn’t too challenging. You just need an Ice Stone. Go from the item menu, use it, and you’ll have the evolved version of the new Pokemon. However, Ice Stones are a bit of a challenge to acquire, especially early on in the adventure. So really, just catching a Cetitan might be faster.
How to Get an Ice Stone in Pokemon Scarlet and Violet
The most reliable place to find an Ice Stone is the Glaseado Mountain area. Which makes sense, as it is the winter Biome in Pokemon Scarlet and Violet, so of course an Ice Stone might be on the ground.
Might is the operative word. They are pretty rare finds. I did find one between Zapapico and the base of the Glaseados, but I used that on an Eevee, so… I was out of luck!
How to Catch a Cetitan in Pokemon Scarlet and Violet
Really, just catching a Cetitan in Pokemon Scarlet and Violet is easier than getting a Cetoddle to evolve. They aren’t uncommon finds in the Glaseado Mountain region. Specifically, I saw a bunch not far from Montenevera while… searching for another Ice Stone. They tend to be surrounded by Cetoddles.
Also, if you’re far enough along in the game, you can find Cetitan in five and six-star Tera Raids.