Easily one of the worst drop rates in-game at being about 15% for being a very sexy DD ammo slot. Aroma NMs pop every 30 minutes in three different locations in Ordelle's Caves, Maze of Shakhrami or Gusgen Mines.
Easist spot, in my opinion, is probably Maze of Shakhrami. The spawns are relatively close to one another unlike Ordelle's Cave. Just keep to it and you'll get yourself one soon enough. ^^