You've now got Pokemon Shining Diamond and Brilliant Pearl in your Switch and you've picked your starter. From here, you may be wondering how you get your hands on the rest. Luckily, there's a way to do it without trading. Here's how.
How to get all Starters in Pokemon Brilliant Diamond and Shining Pearl?
To get all the starts in Pokemon Brilliant Diamond and Shining Pearl, you have to first unlock the Grand Underground.
To do this, you have to go to the underground man's house in Eterna City. It's located next to the Pokemon Center.
Talk to him and he will give you an explorer kit. Use it outside to get to the Grand Underground.
This will only get you to the start of the underground. You have to do a few things to unlock the right area. First, you should complete the game, After, you have to complete the Sinnoh Pokedex and then return to Rowan. There are conflicting reports on which of these gets you the dex so try both. You won't need to trade or cheat to finish the Sinnoh dex.
You will then get the national dex and you can return to the Grand Underground. You can now grab all the starters for the first four Pokemon gens.
Water Types - How to Get Squirtle, Totadile, Mudkip and Piplup
Water types can be found in any of the areas made of water and grass. This may be a little harder to grab than dedicated areas as Pokemon are split between the two types.
Fire Types - How to get Charmander, Cyndaquild, Torchic and Chimchar
You can find the fire Pokemon in any of the fire areas. They may not always appear so you can go in and out of each area multiple times to get them to appear
Grass Types - How to get Bulbasaur, Chikorita, Treecko and Turtwig
You will find the grass type Pokemon in grassy areas. You can find an area that is entirely grass to get a better chance at catching what you want. Just keep pacing around and going into new areas to find whatever you are looking for.