How do you catch the other 2 starter Pokemon in black 2?

Question

I got a Snivy (which is Serperior now) but I want to catch a Tepig and an Oshawott but I don't know where to find or get them.

Answer 1

You can't. You must trade from another Black, Black 2, White, or White 2 for them.

Answer 2

Well, trading would be your only option.
You can obtain them either by the GTS or by trading with a friend