Larvitar is a sought-after Pokémon in any game it is available in, not only because it is popular, but also because it is one of the very few species to have obtained the status of pseudo-legendary—better known as Pokémon that have stat totals that rival the legends.
Thanks to the Grand Underground, players can actually capture all five of the pseudo-legendaries in Brilliant Diamond and Shining Pearl, which was also true for the original generation four titles.
Among the five obtainable Pokémon in that category, Larvitar’s evolution line is the only one that ends on a totally unique type combination, as Tyranitar becomes a Rock/Dark-type. This gives an edge over Dragonite and Salamence directly, while it has some advantages over Metagross too—just avoid Garchomp like the plague.
Unfortunately, you won’t be able to get Larvitar in BDSP until after you complete the main story and become the Champion of Sinnoh.
Once you are Champion, you will be able to encounter Larvitar. But you will first need to complete the Sinnoh Pokédex, so you can gain access to the National Pokédex. This will allow you to visit Ramanas Park to the southeast of Sandgem Town, talk to Professor Oak, and obtain the Poké Radar.
The Poké Radar is a handy device that you can use to encounter rare Pokémon in tall grass on various routes. This is also the only method available to obtain Larvitar in the overworld, as you need to travel to Route 207 directly to the North of Oreburgh City.
Larvitar has a 12 percent spawn rate when using the Poké Radar, so just keep checking those shaking patches in the grass until you nab yourself one.
Additionally, it is available in the Grand Underground as a National Pokédex spawn within the Rocky Cave and Big Bluff Cavern Pokémon Hideaways.
Unfortunately for anyone playing Shining Pearl, Larvitar is a Brilliant Diamond exclusive. So if you want to get your own, you should probably find a friend to trade with. Maybe you can offer up a Bagon in exchange since it is also version exclusive.