Chocolate and Ricin
Ricin, the poison allegedly sent to senators and the White House, is an easy deadly poison to make. The same seed that makes ricin is used to make chocolate.
The stalks are fine. The green leaves are deadly. The poison is oxalic acid, also found in spinach. Food shortages during World War I caused people to eat rhubarb leaves; several people died.
Potatoes and Tomatoes
Both are members of the Deadly Nightshade (belladonna) family. In the case of tomatoes the fruit is fine but the stems and leaves contain tomatine. Potatoes are more deadly. Here the poison is solanine found in leaves and sprouting tubers.
All these plants are completely legal to grow. The deadly parts of the plants are legal to own and transport across state lines (except for raw bitter almonds which must be heat treated). Hemp, quite non-lethal, is, however, extremely illegal.