You can't. You must either trade one in from red/blue/yellow or get one as a gift from oak in Pallet town.
Minor correction; There is no starter gift in the second gen version of these games and I've only heard rumors of it being in the fourth gen version after beating Red, but I know for sure you won't find it in the original.
Trading is pretty easy. If you have a venusaur in an old game that you don't want to restart then you can easily trade that over and breed it with a ditto for a lvl 5 bulbasaur.
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You cannot catch a Bulbasaur in the wild. You must defeat Red on top of Mt. Silver. Then, you can go to Professor Oak and choose the bulbasaur.
Bulbasaur is a starter for the Johto Region, so you cant catch him in the Sinnoh.
you do not catch it . you can bulbasaur when you get it from proff oak
you can not catch bulbasaur you have to pick him as your starter Pokemon
No. Bulbasaur is a Starter Pokemon from a different region besides Sinnoh, so you will have to trade somebody with a different Pokemon game for it.