The Super Rod in FireRed & LeafGreen is in the same location for both games: inside the fishing guru's brother's house on Route 12. You can get here after you obtain the Pokeflute and wake the sleeping Snorlax on Route 12.
The fastest routes are to either fly to Vermilion and head east all the way to Route 12, or fly to Lavender Town and just go south all the way. Below is a screenshot of the house to look for, plus a link to a tutorial on how to reach this house and get the rod:
(Source: Guide Strats)
