In Pokémon FireRed and LeafGreen, you can get the VS. Seeker by talking to the purple-haired NPC in the Vermilion City Pokémon Center. Talk to her and she’ll tell you how much she likes fighting trainers over and over again, then she’ll give you the item for free.
You’ll first get access to the VS. Seeker after clearing the S.S. Anne, getting Cut, and defeating Misty.
After completing the prerequisites, the next thing to do is travel to Vermilion City.
Step 1: Reaching Vermilion City
To get to Vermilion City, just head to Diglett’s Cave right below Pewter City. As you can see, there will be a tiny tree in your way, so you will need to be able to use HM 01 Cut.
Once inside, you can just walk through Diglett’s Cave with no issues or barriers.
Alternatively you can travel to Vermilion by going south from Cerulean City. This will lead you through an underground tunnel that connects to Route 6. South of that route is your entrance to Vermilion City.
Step 2: Getting the VS. Seeker
Once you’re in Vermilion City, just walk to the Pokémon Center and talk to the NPC with purple hair (pictured below).
She’ll tell you how much she loves to battle trainers many times (who doesn’t?) then hand you the VS. Seeker.
She won’t even make you answer a Yes or No question for the item.
How To Use The VS. Seeker
The VS. Seeker allows you to battle trainers that you’ve already defeated in the past.
Using it is simple:
Open up your bag and use the item around trainers that you’ve already defeated. Exclamation marks will appear above the trainers’ heads and you will be able to talk to them to initiate a battle.
The VS. Seeker is a great way to farm EVs and train your Pokémon through battles.
Of course, you can’t spam the VS. Seeker nonstop. After you use it, it’ll need to charge for a little bit before you can use it again.
Conveniently enough, the game will tell you the exact number of steps that you need to walk in order to recharge the VS. Seeker.
So all you need to do is walk around a bit and you’ll be able to challenge anyone your heart desires.