You can either enter Route 12 by going east from Vermilion, through Route 11, or you can go south from Lavender Town. For the purposes of this walkthrough, we will be going south from Lavender. So fly back there now, and go south.
Like as before, right where it was laying is a hidden Leftovers. You have to use the Itemfinder on the exact tile he was on.
Right below the Snorlax is a house, where the final Fishing Guru is. He gives you the best fishing rod, the Super Rod! Now you can go back to previous places you've been to catch some stronger, rarer Pokémon, such as Dratini in the Safari Zone. He also asks if you catch a large Magikarp (there's no way to tell if it's large or not), and if you do, he'll give you a Net Ball, a type of Poké Ball that increases the chance of capturing a Bug or Water-type Pokémon.
Keep heading down and you'll see some more trainers.
There is another group of trainers waiting to battle you. However, before battling them, cut down the nearby tree and head left a little and you can find TM18 (Rain Dance).
If you head west through the gate to Route 15, go up the stairs in the gate and you'll find the last of Oak's aides. If you have registered at least 50 Pokémon in your Pokédex (if you went through the Safari Zone enough, you probably do), he will give you an Exp. Share. Make a Pokémon hold it, and it will receive a share of the EXP points earned without having to battle. Also, if you look in the left binoculars, you can see Articuno flying towards the sea. Keep going left and you'll be back in Fuchsia City.