Atropa Belladonna, or Nightshade as its popularly known as; is found in western Europe naturally. Associated with the planet Saturn and extremely deadly. Beings it's a poison and we aren't in the habit of promoting poisons or the use of them in spellwork around here, I'm not going to suggest where to purchase it or recommend its use in spellwork.
However a cousin of nightshade that is much safer to work with is Mandrake. Also associated with the planet Saturn, and a native plant of southern Europe. Though it is still considered a baneful plant and ingestion is a very bad idea, it is easier to come by. Seeds and even dried plant parts can be purchased online and from some local botanists. (Depending on your area online purchase might be the way to go.)
Depending on your path, practice or work a couple neat tips on Mandrake is that it is sacred to Hekate, associated with Diana, Aphrodite, and to Mercury (apparently because of the shape of the root). Magickally speaking its been used in flying ointments, poppets, and even carried for protection and love.
Now given that Belladonna s typically used in death and healing work, Mandrake will substitue nicely on the healing side, though you might have to find a completely different herb for death work. If you are just trying to honor the Roman Goddess of War, Bellona I might suggest an altar dedicated to Bellona using things such as milk, honey, wine, perfume, depictions of animals, birds, blood, etc.
Either way you go I wish you luck in your endeavors, and strongly urge you away from using poisons and deadly herbs in your practice.