In Cianwood City, there is a Pharmacy where you can get the SecretPotion. If you haven't already, you need to go to the Lighthouse and talk to Jasmine about her sick Ampharos, otherwise you can't get the SecretPotion. I assume you already have seeing you know what it is, but there is the chance you played the original Gold/Silver/Crystal and tried to skip the Lighthouse Part 1.
The area colored in orange is where Cianwood City is. Surf down from Olivine to get there.