Belladonna is better known as deadly nightshade, or just nightshade. If you watch game of thrones it is the type of poison cerci uses to almost kill her son when she thought they were about to die, if I am not mistaken it is also the poison used to kill joffery.
Anyhow nightshade has a long history with witchcraft, but as mentioned earlier can easily kill you. There is tea made from nightshade online you can order, it promotes sleep and visions. A lot of relaxation and sleep teas have a hint of belladonna.
However just an ounce more than you need can have bad affecrs. It would be best to talk to sone one who studies herbs, and explain to them what spell you are trying to cast and if you have to ingest any and how much if so, they can tell you the possible side affects.
When using nightshade make sure you measure properly, people also used to burn nightshade and similar herbs to provoke visions, but if you are doing that you should still be careful as ingesting literal poison into your lungs can't be too healthy.