Litten is not available in Switch Pokemon games without DLC, Trade, transfer, or event.
Your best option is to trade from another player who has the game, or trade within Pokemon Home. You can try other designated trading forms and ask players if they have a spare Litten for trade.
If you can't set up a trade with anyone, download Pokémon Home on your mobile device and try wondertrading.
Lastly, you can try playing Pokemon Go and wait for a boosted spawnrate event, and transfer litten to Home and then into your game.
Here's a comprehensive list of all the methods available to obtain Litten. I know you said no DLC or Gen 7, but I want to make a comprehensive list with all the options.
Trade / Transfer from the following games:
- Transfer from Sun/Moon/UltraSun/Ultramoon
- Transfer from Pokemon GO
- Trade/Transfer from Pokemon Home
DLC Options
- SWSH Expansion Pass: Reward for finding at least 100 Alolan Diglett (if the player chose Scorbunny)
- The Hidden Treasure of Area Zero in Canyon Biome,: Chargestone Cavern (after boosting biodiversity in the Canyon Biome)
Note: Litten is completely unobtainable in LGPE, BDSP, and Legends: Arecus
- Litten was made available in SwSh though Wild Area News in 2021. this was limited-time only.
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