There is a small chance to obtain Rivals in boosters, and through dungeon bosses. Additionally, you can purchase them in the store for Aether Coins or as part of a bundle. A new legendary is added every month as part of the Rival Pass.

• You receive 1 copy of a legendary Rival when you purchase a Rival Pass.
• The Rivals Pass gives you access to a Legendary Dungeon, where a second copy of that Season’s Rival can be earned for no extra cost.
• Each Season has a Legendary Chest containing a Rival that can be earned through AP. Each Season’s Rival Pass includes an additional Legendary Chest that can be earned through AP.
• There is a 4% chance of finding Rivals as bosses in Quick Crawls.
• There is a 10% chance of finding Rivals as a dungeon bosses in Elemental Dungeons.
• If you already own a Rival, they will occasionally appear in the shop for 40,000 coins.
• You can occasionally buy Rivals in special bundles.
• Rivals can be found in Legendary Chests.
  
• Rivals can be earned as a reward on the ladder (at the end of Training 2, Julesvale, and Diamond 1).
• Rivals have chances to appear in several in game booster packs and chests – 0.6% chance in Elemental Dungeon boosters, a 1% in Aether chests, a 2% chance in standard boosters, a 4% chance in giant boosters (125 orbs), a 6% chance in epic boosters.
  

(Updated January 24th, 2021)