Click here for a map w/ items.
That map should help you find the Spooky Plate. It looks fairly near to the entrance, you can see where to go on the map. First floor.
As for the Splash Plate, it cannot be found in the Abyssal Ruins. It is found in Undella Bay near Ace Trainer Summer.
Anyways, to get them just study the map, and follow the passages as fast as you can until you get the plate. I recommend just getting one Plate each run unless they are really close.
Hope I helped! :)