Pokémon Brilliant Diamond and Shining Pearl are remasters of the original games and are fairly faithful to their predecessors. One thing that holds true is the choice of starters, ie Chimchar, Turtwig and Piplup. But some coaches prefer original entries and may wonder if they're catching them.
You can find Charmander, Squirtle, and Bulbasaur in Pokemon Brilliant Diamond and Shining Pearl in the Great Underground. But first you will need to have acquired Professor Rowan's National Pokédex. After that, you can find Gen I starters in various Pokemon Hideaways. These are:
Related: How to Catch Gible in Pokémon Brilliant Diamond and Shining Pearl
- Charmander – Volcanic Cave, Sandsear Cave and Typhlo Cave.
- Carapuce – Grotte de Fountainspring, Riverbank Cave et Still-Water Cavern.
- Bulbasaur – Riverbank Cave, Grassland Cave, Still-Water Cavern et Sunlit Cavern.
If you don't see the Gen I starters in these various locations, just exit them and re-enter them until you see Charmander, Squirtle, or Bulbasaur. Once they spawn, simply interact with them to encounter and catch them as you would any other Pokemon. Good luck there trainer!
For more information on Pokémon, check out What are the Shining Odds in Pokémon Brilliant Diamond and Shining Pearl? and How to get Eevee in Pokemon Brilliant Diamond and Shining Pearl on Pro Game Guides.