Laportea canadensis - Sandy Mush Herb Farm (excellent Garden Watchdog rating) sells the plant.

Urtica dioica - many sources out there. Garden Medicinals and Culinaries (good but limited Garden Watchdog rating) offers seeds. Sandy Mush Herb Farm has plants.

I'm not sure where you live but you could grow Australian gympie-gympie the most painful plant in the world,or opuntia with its spines that penetrate leather gloves or poison hemlock with its horrible alkaloids. Aralias are rather wicked with their trunk thorns.The most wicked plants to me are things like dandelion,oxalis,creeping spurge,dayflower,shepards purse,chickweed,ect,ect.

Thank you @brandon for the options. @jaybird Would you know where I could get Gympie gympie seeds? :D

Gosh,I don't. I'm in Illinois zone 5 so I wouldn't even attempt growing it,but if I was in a warmer climate I probably would.I have stinging nettle which is related to gympie-gympie but not quite as painful.It's a host plant for Red Admiral butterflies.If I happen to brush a leave I feel a burning,numbing sensation that can last a whole day,but they say the pain from gympie-gympie can last month's or years.Horses have been reported to jump off cliffs because the pain was so bad.

Very true. I actually moved from Indiana Zone 5b (transplant from New Jersey Zone 7a prior to that) almost two years to Los Angeles Zone 11. A lot has changed horticulturally. Yeah, the gympie-gympie is dangerously intriguing! I have heard of the stories, especially the ones in which it was dangerous to even breathe in the hairs (trichomes)! Fanatic botanic!