I can't find anything about this on Google, Bing or Stack Exchange. Does Oko, Thief of Crowns come with a planeswalker deck, or separately? By separately, I mean in booster packs and stuff like that.
No, Oko, the Trickster is the version of Oko in the planeswalker deck, not Oko, Thief of Crowns. Planeswalker deck planeswalkers are different from the planeswalkers in their set and are numbered above the number of cards in that set, but still considered part of it for standard/modern legality. The planeswalkers in these decks are generally considered to be overcosted, less powerful and less useful than their set counterparts.
The planeswalker deck planeswalkers to date:
- Kaladesh: Chandra, Pyrogenius and Nissa, Nature's Artisan
- Aether Revolt: Ajani, Valiant Protector and Tezzeret, Master of Metal
- Amonkhet: Gideon, Martial Paragon and Liliana, Death Wielder
- Hour of Devastation: Nicol Bolas, the Deceiver and Nissa, Genesis Mage
- Ixalan: Huatli, Dinosaur Knight and Jace, Ingenious Mind-Mage
- Rivals of Ixalan: Angrath, Minotaur Pirate and Vraska, Scheming Gorgon
- Dominaria: Chandra, Bold Pyromancer and Teferi, Timebender
- Core Set 2019: Ajani, Wise Counselor, Tezzeret, Cruel Machinist, Liliana, the Necromancer, Sarkhan, Dragonsoul, and Vivien of the Arkbow
- Guilds of Ravnica: Ral, Caller of Storms and Vraska, Regal Gorgon
- Ravnica Allegiance: Gideon, the Oathsworn and Jace, Arcane Strategist
- Core Set 2020: Ajani, Inspiring Leader, Mu Yanling, Celestial Wind, Sorin, Vampire Lord, Chandra, Flame's Fury, and Vivien, Nature's Avenger
- Throne of Eldraine: Rowan, Fearless Sparkmage and Oko, the Trickster.