Where can I find Nightshade?

Go to a Mage in the Arcane University and ask about it and he/she will send you to a count, that count will send you to a Witch. That witch will ask you to do a quest in order for her to make the cure.

3 Nightshades are laying on the table in the Chorrol Mages Guild library (2nd floor). Another 2 are laying on a slab in the Night Mother's crypt, but you can only go here as part of the last quest for the Dark Brotherhood.

There's also two Nightshade and two Human Hearts in Fort Farragut, Lucien's Room. Easiest to get, probably, as no enemies infest the fort's exterior (the road is another thing), and you can also loot the entire place for 10 Poisoned Apples, some leveled jewelry, a few potions and poisons and some nice books and food items. And there's a free bed and no pesky mages watching you steal.

You can find nightshade mostly in alchemic shops.There is mostly in Skingrad All Items alchemical

There are some nightshade plants in the Arcane University as well.

In addition to KingPhill1234's comment there is an area east (towards bravil) and slightly north of the priory of the nine that has alot of nightshade. This area is still close enough to the priory so that you can see it.

There is nightshae behind mages guild in skingrad also for any ingedient checkk main ingredient in imperial city

The Main Ingredient is the only reliable Alchemy shop that will carry Nightshade on a regular basis. Others can, but it's a matter of luck with them. As for where it can be found, well you can find it all over the West Weald should you feel adventurous. Hope this helps. Have a good day!

I've found alot scattered in chests in Ayleid ruins as well.