Trading is a pivitol part in Pokémon to get you to complete your Pokédex...in the game there are several trade areas where you are capable of trading for in-game Pokémon. They always come attached with a bit of Mail and are always have the same EVs and levels as the Pokémon you sent over. So if you sent a Lv. 5 Skitty over, you'ld get a Lv. 5 Meowth:

Fortree City

This trainer in a house on Fortree City is looking for a Volbeat

Pokémon You Send Pokémon They Send
#313 Volbeat
 
#311 Plusle
Wood Mail Attached

Pacifidlog Town

This trainer in a house on Pacifidlog is looking for a Bagon

Pokémon You Send Pokémon They Send
#371 Bagon
 
#116 Horsea
Wave Mail Attached

Battle Frontier

This trainer in a house in the Battle Frontier is looking for a Skitty

Pokémon You Send Pokémon They Send
#300 Skitty
 
#052 Meowth
Retro Mail Attached

 
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