To get the master ball you need to do a series of things, start by doing the following: Get through Rock Tunnel which requires you to know the FLASH and CUT HMs. Go straight to Celadon city. Visit the Celadon Mansion, there should be a guy next to it facing away from you. When you talk to him he will say, the nice lady in Celadon Mansion gave him tea but he burned his tongue with it. Now go inside the Mansion and have the rich lady give you the tea. With that tea you can open the way to Saffron City later on. If you find a way to go through the back door of the mansion, you can find an Eevee inside an item ball, on top of a table!