The Sims 3 Guide - Sunset Valley
Collectibles - Seeds
Seeds can be found across Sunset Valley, though particularly by Landgraab Industries Science Facility and usually close to roadsides. The most lucrative seed to find is that of the Death Flower. The flower itself can be harvested once from Pleasant Rest Graveyard, or found by chance while fishing. It allows you to buy more time from the Grim Reaper should your Sim die. It is not valuable in itself however, unlike the Money Tree, which can fetch 800 Simoleons per 'money bag' produce if in perfect condition.
The Death Flower, by the entrance to Pleasant Rest Graveyard.
Of course, to make use of the seeds, plant them in your garden and look after them well. You can then sell their fruit.