there are some trainers who have bulbasurs at the battle frontier.
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You cannot find Bulbasaur in Pokemon Platinum. you can only get a Bulbasaur through trading from Pokemon Leaf Green or Fire Red. You can even use an AR to get Bulbasaur in Pokemon Platinum. Sorry I don't remember the code. Have fun and good luck.
your rival in the game
The trainer on Route 209 has Cleffa in Pokemon Platinum. Cleffa is a fairy type Pokemon and can evolve into Clefairy.
Bulbasaur is a starter for the Johto Region, so you cant catch him in the Sinnoh.