You can find both of them in Pinwheel Forest or Routes 10, 16, and 18.
According to Serebii, Throh's rarity in Black is 5%, while in White, it is 10%. Then Sawk's rarity is 10% in Black and 5% in White.
Originally Posted by Chris2K
Anyone been able to get a Sawk/Throh in shaking grass yet?
I've only found Sawk (in Black) in the grass two times on the outside of Pinwheel Forest twice while training, but they each appeared in a double wild Pokemon battle.