Pokemon league champion Cynthia will give it to you when you run into her at lake valor(near pastoria and sunyshore)
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you go down to the house next to the Pokemon center and inside is this guy, talk to him and he will give a secret potion.
Cynthia will give it to you.
You get the secret potion by talking to the pharmacist in Cianwood City. His house is in the south, south east of the gym.
cianwood city
You get the potion from Cynthia at Valor Lakefront. Then go to route 210 where the psyducks are.