Venasaur is one of the rarest and strongest Pokemon in Pokemon GO, making it easily one of the most sought after. It can be seen in the wild, albeit rare, but by getting more Bulbasaur Candy and evolving the basic Pokemon, you can add one to your collection.
You will need to evolve Bulbasaur into Ivysaur, and then the latter into Venasaur. For this you will need a good deal of Candy. There are three simple ways to get more Bulbasaur Candy.
First of all, you should catch as many Bulbasaur 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 water types, grass types can be found in most location, from cities to parks. Often, they are more common in grassy areas, like fields or parks.
Secondly, if you have caught a lot of Bulbasaur or Ivysaur, you can transfer them to the Professor and get one Bulbasaur 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.