I'm trying to fulfill the side quest for Injun Blackbriar where she needs ingredients. I have bought some nirnroot and nightshade, but it isn't enough to reach the goal because vendors rarely have it. Are there a few places where I might be able to collect these that you know of?

There are a few nightshade plants in Solitude, and nirnroot are found near water. They also like to yell at you, so there's that.

Lots of Nightshade in the cemetery in Falkreath as well as some right there in Riften- check the tombstones behind the Hall of the Dead.
There's a motherlode of Nirnroot- Otherwise, just travel the waterways at night and you should spot plenty.

there are hell lots of nightshades in solitude, Then check falkreath and riften graves. The archmage room also has 2 or 3

The salt marshes near Morthal abound with both plants. But wear your resist poison gear when you go gathering flowers!