Go to saffron in kanto and it is somewhere behind the pokemon center
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in the silph co. building.
You can't.
In HeartGold and SoulSilver, you can access the 1st floor of Silph Co. as soon as you enter Saffron City however you can't go into the elevator in order to get to the 2nd floor unless you have a Rotom as your lead Pokemon.
get all the bags then
In Pokemon Gold, Team Rocket has been defeated and the Silph Co. office building is mostly empty. The player can go inside and find an Upgrade item however.