Where do you get seviper in Pokemon SoulSilver?

Answer 1

Seviper is in the Safari Zone. Specifically, it is in the swamp area. In the swamp area, you have to put certain items or blocks to attract seviper.
Go to the Safari Zone and switch the Marshland in so you can go to it. Place 18 Plains objects and wait 10 days.

I suggest changing the time on your DS to 23:59, then starting your Soul Silver and waiting until the clock says 0:00, thus, it will be a new day. Then save and do it again ten times, then look for you Seviper. Don't change the date; it will change on it's own.

Or, if you don't want to do this, trade on the GTS or transfer from a GBA game.

Hope this helped.~