Usually you can find a lot in Kishibe Village. Just switch realms whenever you find black lotuses. I need 15 diamonds for this one crafting quest and I can't buy them at the bazaar. Can someone tell me where to find some?
Both black lotuses and diamonds can be transmuted from various more common reagents. If you wish to transmute, you have to buy the recipe from one of the recipe vendors. Regardless of how you go about it, collecting/making black lotuses and diamonds will be time consumptive as they are very rare reagents.
I'm certain that one of the Wizard 101 fansites has a good guide to recipes and where to find them. I'd tell you myself, but I'm not sure the moderators will allow this sort of information sharing under the "spoiler" rule for posting.