Any broc flower will do. Near the campfire where Sunny is waiting for you is another one. Go up the hill past the broken radio tower there's one up the hill just past the base of the tower.
There are also tons of them above I-15 in the wasteland between Goodsprings and Primm. If you follow that ravine that cuts I-15 up the hill you'll find a pond surrounded by broc flowers.
However, that quest should spawn the broc flower in the graveyard: maybe its an issue with old cache files you could try clearing your cache too. The Xander Roots and Broc Flowers that get marked by the quest are only enabled if you start the quest, so if they are not appearing it's probably an old cache that got corrupt.