Go to a beauty spa and take their supply of Botox. Boom incredibly toxic bioweapon. Or grow some Castor plants, the beans contain ricin.
IDK much about Botox as a weapon, but can you use it to deploy broadly? With ricin, that's just another poison, so how's that better than rat poison?
You could trivially cause enormous harm by sabotaging electrical distribution, water treatment, hospitals, airports, etc. Go to any food manufacturer/distributor and contaminate the products with listeria or such.