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If you have Realm of Magic, the vendor in Casters Alley sells an always-changing stock of magical plants and sometimes death flower is for sale there.
If you have a skilled gardener you can also get a death flower by grafting plants. You can look online for the recipe. I think you have to graft 2 or 3 different plants first before you can graft the results to make the death flower. It takes some time. Skilling up fishing might be faster depending on how long it takes you to come up with a death flower after reaching level 8.