Go to Celadon city and enter the Celadon Mansion. On the first floor of the mansion, there should be an old lady who will give you tea.
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celadon in building with tea
In Celadon City you need to go into the Celadon Mansion (Front Door) and talk to the little old lady babysitting the Pokemon she will give you the tea
The Tea item is exclusive to Pokémon FireRed and LeafGreen. It's not possible to obtain the Tea item in Pokémon Diamond or any version that is not FireRed or LeafGreen. The only way to have the Tea item in Pokémon Diamond is via a hack.
In Pokemon firered you can't, but in Pokemon leafgreen you can get it then trade it to Pokemon firered.
To get Pokemon firered on PC download a vba and Pokemon firered ROM