There are maps for NM spawns out there, but I hate to say it, it'll likely be well-guarded information. The idea is if we went around just telling everyone where the NMs were, how often do you think we ourselves would get to hunt them? The answer is never. NMs are already hunting to the endangered species list, why would we want to make it even more difficult to get them.
I will, however, give up a few NM pointers.
When hunting for a NM, to make it spawn you have to kill it's place-holders. A place-holder is a monster of the same type as the NM, and spawns in the same place. When the place-holder is killed, it take a little time for a creature to respawn in that spot, from 6 to 16 minutes. When the creature spawns in that spot, the system rolls randomly to see if the creature is spawned or a NM is spawned in its place. When hunting a NM, there's no point in killing everything in the area, just concentrate on the monster type matching the NM.
For some creatures, the NM spawn location is set to the exact place, but for the majority of them, it is set to an area sometimes spawning several grids. Quest NMs will have the exact same spawn locations, are set on an exact spawn clock. Treasure NMs will not usually be so predictable, requiring many kills before you hit the random spawn for the NM. Some NMs, however, I've seen spawn one within minutes after it had just been killed, or hours apart like in your instance. To make matters worse, a lot of the times the treasure item you're looking for may not be a definite drop, so it's better to have one or more Treasure Finder skills in your group.
There are a few NMs, that are like the Quest NMs in that they have a spawn clock. These are few and far between, but an example is Baldurf Bloodtear in Konschtat. NMs like this will take days, sometimes longer, to spawn, and I have seen a site where the clocks are posted. Unfortunately, I don't know Japanese, so translating these sites to get the information you want can be tedious.
Here at Allakhazam, the bestiary section contains a good many of the NMs you will want to hunt, and once you get the hang of it, hunting them isn't that hard, just time consuming. For more details, or for NMs not found on Allakhazam, all I will say is that the information is out there, I hope you're patient and/or can read Japanese.