Where can i find the Death Flower?

If you have a spellcaster they have a chance of creating one with the herb spell.

I've gotten death flowers via fishing. The river in Willow Creek behind Bargain bend and near the hidden cave at the park in Oasis Springs can give you rare items like that. You need a super high fishing skill, though. Otherwise, grafting or the potions store in ROM. Once I received it as a present to one of my sims on their birthday. I also don't reccomend purchasing rare seed packets. They NEVER pay out.