You might not be able to find a Bulbasaur or Squirtle capable of Gigantamaxing in the wild, but that doesn’t mean you can’t still find the one you didn’t pick the Isle of Armor’s main story.
Like with pretty much every other Gigantimax encounter in Pokémon Sword and Shield, you will need to capture the starters after completing a Max Raid Battle.
There are specific areas on the Isle of Armor that you can find Max Raids featuring Gigantamax Blastoise and Gigantamax Venusaur, though you will need to use some system manipulation to reliably get those encounters.
Starting with Gigantamax Blastoise, you will need to head back to the train station where you originally arrived on the Isle of Armor. From there, head south out of the Fields of Honor into the Workout Sea, where you will see an island with a large, arching rock formation.
Once there, head to the middle of the island and right next to a Roaming Pokemon you will find the Raid Den you are looking for.
Just as Blastoise is found on the ocean, Gigantamax Venusaur will be found deep in the Forest of Focus. When entering from the side closest to the Master Dojo, keep to the right hand side of the path (avoiding the bridge) and you should come across a small pond with a Raid Den placed next to a tree inside.
Unlike with the Blastoise den, there will be a chance to encounter regular Venusaur as a raid too, so make sure you can differentiate between the shadowed sprites in order to avoid a mixup.
Both Gigantamax Blastoise and Gigantamax Venusaur can only be found in five-star raids, and here is how you can ensure you encounter one and don’t waste your Wishing Pieces.
First, you will need to change your text speed to either Slow or Normal depending on how fast you plan on reading and reacting to the text of the Raid Den before saving the game. Using your Wishing Piece, spawn a raid, but before saving the game as prompted, hit the Home button.
You are looking for the rare, purple Raid Den spawn, so if the den is red, close the game before it saves your progress. The purple beam raid has about a 10 percent spawn rate, so you might be doing this a few times before successfully getting it right.
Once you do have the rare raid spawn, you can return your settings to normal and save the game again.
If the five-star raid does not feature either Gigantamax Blastoise and Gigantamax Venusaur, you can hit Invite Others (make sure you are offline first,) change your Nintendo Switch’s date to the next day in the Time and Date settings, and then quit out of the search. This will eventually get you to the right Pokémon because it resets the encounter without resetting the raid itself.
You can repeat this process as many times as you need to until you get both of the new Gigantamax Starters.