In newer Pokémon games you can find both of the items needed for evolution, which allows you to easily get both.

Ruby, Sapphire, and Emerald – In order to get the item, you will need to be able to Surf and Dive. Travel to Route 108 and Surf to the Abandoned Ship. Use Dive to access the second part of the Ship. This will take you to a room with 5 locked doors. The keys will be visible as sparkles for a moment when you enter the room. Find the Scanner in Room 2, and take it to Captain Stern in Slateport City. Choose between the Deep Sea Tooth or Deep Sea Scale as your reward. Diamond, Pearl, and Platinum – Certain wild Pokémon will be holding these items. This means that if you capture them, they may have the item in their possession. You can then release the Pokémon back into the wild after putting the item in your Bag. You can also try to use the Thief, Covet, or Trick skills to steal the item. You can find Deep Sea Scale on Chinchou, Relicanth, and Lanturn. You can find Deep Sea Tooth on Carvanha and Sharpedo. Black and White – You can find the Deep Sea Scale on Route 13, and it is also held by wild Chinchou, Lanturn, Gorebyss, Relicanth, and Blue-Striped Basculin. You can find the Deep Sea Tooth on Route 13 and Route 17, and it is also held by wild Carvanha, Sharpedo, and Red-Striped Basculin. Black 2 and White 2 – The Deep Sea Scale and Deep Sea Tooth can be purchased from the Antique Shop on Join Avenue, located between Route 4 and Nimbasa City. To open the antique shop, choose the response “I would want to collect unique items and surprise everyone”. In order to get the right items from the Antique Shop, Pokémon Ranger Atilio will need to be the shopkeeper. When Atilio is running the shop, purchase any of the Box items for a chance to get the Deep Sea Tooth or the Deep Sea Scale. You can also find the Tooth on wild Carvahna, Sharpedo, and Red-Striped Basculin. You can find the Scale on wild Chinchou, Lanturn, Gorebyss, Relicanth, and Blue-Striped Basculin In White 2, the Tooth and Scale can also be found in the White Forest. X and Y – Both the Deep Sea Tooth and the Deep Sea Scale can be found in the Azure Bay.

This will take you to a room with 5 locked doors. The keys will be visible as sparkles for a moment when you enter the room. Find the Scanner in Room 2, and take it to Captain Stern in Slateport City. Choose between the Deep Sea Tooth or Deep Sea Scale as your reward.

You can find Deep Sea Scale on Chinchou, Relicanth, and Lanturn. You can find Deep Sea Tooth on Carvanha and Sharpedo.

In order to get the right items from the Antique Shop, Pokémon Ranger Atilio will need to be the shopkeeper. When Atilio is running the shop, purchase any of the Box items for a chance to get the Deep Sea Tooth or the Deep Sea Scale. You can also find the Tooth on wild Carvahna, Sharpedo, and Red-Striped Basculin. You can find the Scale on wild Chinchou, Lanturn, Gorebyss, Relicanth, and Blue-Striped Basculin In White 2, the Tooth and Scale can also be found in the White Forest.