Pokemon-go – I picked the wrong starter, is there any way to change it

Small bodies of water would be enough for a squirtle and small patches of grass would even be enough for a bulbasaur. A buddy of mine told me he got charmander in the middle of the city so it seems he's easy to catch.

Now the starter Pokemon are quite rare to appear so it will take some time for them to spawn.

Borrowing from @Codestrife's answer here

The rustling leaves usually indicate that there are pokemon near that area. Going and walking around near that area gives you a higher chance to encounter Pokémon.

Keep an eye out for rustling leaves, hinting at where critters may be lurking and simply hang out in a suspected region for the increased chance of an encounter. - Source

Therefore, the reason you don't always see Pokémon in rustling grass is because there is only a higher chance to encounter them there and not a guaranteed chance of encountering them. You just have bad luck when you don't encounter them.