In Pokemon Legends: Z-A, you basically get one of the Kanto starters for free, but then need to wait until Wild Zone 20 opens up to be able to catch Bulbasaur, Charmander, and Squirtle in the wild. Which is fine. Especially since you get access to Side Mission 022 to get the first one early on. However, if you’re going for a character for its Mega Evolution, one isn’t the “best” pick since you need to wait for its Rogue Mega-Evolved Pokemon fight.
How do I get the Kanto starters Bulbasaur, Charizard, and Squirtle and their Mega Stones in Pokemon Legends: Z-A?
Your first chance to get one of these three is when the Side Mission “A Call from Mable” appears. This tasks you with checking in with the professor at the Pokemon Research Lab right near Wild Zone 1 and Vernal Avenue. It comes up right after you reach Rank V in the ZA Royale. Here it is on the map, in case you can’t remember where she works.

Head up to visit her on the third floor. An event will trigger and you’ll see a Bulbasaur, Charmander, and Squirtle are standing there. Mable will explain she was tasked with finding a trainer for one of them. Pick one, and you’re done. Note that these will be in Wild Zone 20 eventually too.
As for the Mega Stones for the Bulbasaur, Charmander, and Squirtle evolutions, three of the four are available right away for the Kanto starters in Pokemon Legends: Z-A. The Blastoisinite, Charizardite X, and Charizardite Y are 100,000 each at the Stone Emporium on Vernal Avenue, just up the street from Mable. However, you need to beat the Rogue Mega Evolved Venusaur in Main Mission 29 to get that one.
Here’s where you’ll find that store:

Pokemon Legends: Z-A is available for the Switch and Switch 2.