Trading is one way that you can complete the Pokedex. you can also use trading to get other Pokemon that you just want like legendary or other really cool Pokemon.
How to Trade There are many ways to trade
1. You can go to a Pokemon Center and go to the trade place and trade via two people in all Generations
2. You can go to the world trade center in Jubilife City and trade with people around the world in Genaration IV games.
A trainer trading a Mantyke for a Turtwig
3. You can't trade over Wi-Fi in Gen I-III. You can only trade in Wireless Clubs.
4.Also Find in Buildings