Blastoise is one of the rarest and strongest Pokemon in Pokemon GO, making it easily one of the most sought after. He can be seen in the wild, albeit rare, but by increasing the amount of Squirtle Candy you have and evolving the basic Pokemon, you can add him to your collection.
You will need to evolve Squirtle into Wartortle, and then the latter into Blastoise. For this you will need a good deal of Candy. There are three simple ways to get more Squirtle Candy.
First of all, you should catch as many Squirtle as possible. If you know where to find them, go there and catch as many as you can because each one you catch will give you three candies. Unlike fire types, water type Pokemon tend to stick to particular areas, which is unsurprisingly near water or the coast.
Secondly, if you have caught a lot of Squirtle or Wartortle, you can transfer them to the Professor and get one Squirtle Candy for each. Do this with all but one of each Pokemon to get as many candies as possible from transferring, whilst leaving yourself Pokemon to evolve.
Finally, you can hope to find the starter Pokemon in eggs. Eggs will often award you large amounts of Candy along with a Pokemon, so keep incubating those eggs and keep walking.