If you need to know more specifically where to find Black Lotus just do a search of this forum there are lots of threads about it. Mooshu is the world to find it. You can also Buy it at the bazaar or buy the transmute recipe from Toshio in Mooshu and transmute it.
There are many threads on this board (use the search) that detail how people collect them, and their favorite areas to collect. A good 'run will take about one minute, have 4-6 spawn spots, then shift realms, and redo. Please note, what ever you do, don't try collecting during peak player periods, it is near impossible...log on at low player times.