1) Earth plates > First one at 0,5 in 3rd lv. Dungeon. You need to place water plate which drop from Mimic at 0,5 in 2nd lv. dungeon to fill water in a gap and then you need to swim from 0,7 in 3rd lv. Dungeon across a gap to get a plate. > Second one at 7,6 in same lv. dungeon. You need to place 1st earth plate at 6,8 in 3rd lv. Dungeon to open a way.
2) Air Plate > Left-half from 2nd lv. Dungeon at 2,3. Then, go back to Rat king and talk about a half plate with Tao. > Right-half from talking with Rat king about half plate. > Full Air plate from combine Left and Right half plate. Check on inventory>>Misc.>> click 'use' at any half plate to combine.
3) Water plate > Drop from defeating a mimic (event droping) in 2nd dungeon at 10,8 (always encouter with Mimic), 8,10 and 10,10 (randomly encouter, sometime you can get useful items from here like some weapons, equipments, potions, bombs, money, etc.)
4) Fire plate > If I remember correctly, there still no way to get that.