How do you get tangela on Pokemon Pearl?

You can catch Tangela at mt. coronet top if here is a Tangela swarm. You can go every day to your rivals sister and she says which swam there is that day.

Or after beating the elite four and finishing the Sinnoh pokedex you can import it from other GameBoy Advance Pokemon games such as Emerald, LeafGreen, FireRed, etc. using Pal Park.