Two ways:
Trade
Catch

How 2 chase this pokemon:
Go 2 the lake near ur house.go 2 the middle,and talk 2 it.it will dissapear,and the professer will come and tell u 2 use a pokemon tracker app and it's playing a game with u.Find the ultra repels that block about 350 steps(unless it's near ur area) and find a pokemon that can know sleep, moves and/or mean look.DON'T FLY,IT MAKES IT MORE FAR AWAY!Make sure the pokemon is very fast,and whatever u do, DON'T THROW A MASTER BALL OR KILL IT!(Unless u have a lot of master balls,don't think about it)Chase the pokemon,get in it's area,walk around on the grass,cave,water,or whatever,and use the mean look 2 keep it there.When it's low HP,throw a pokeball at it.(I prefer ultra)If it flees,repeat but DON'T KILL IT!If u meet it again and it's low HP,try using a time ball.
If u still need help,ask me.