RPG Insanity

I've been trying to find the place where metals spawn but so far nothing. Anyone knows where it is?

There isn't one.  There is LV200 island, LV500 island, LV800 island, and the Dead Sea.  In the post-game more areas open after talking to Enah at the Guild.

Metal monsters can sometimes be found in tbe 500 area.

Yeah, the metal monsters are a pain to find this time. Here's where I encountered them (from strongest to weakest):

Metal Dragon: Nibill False Tomb
Metal Behemoth: Nibill False Tomb, level 800 area
Metal Ceratops: Nibill False Tomb, level 800 area, level 500 area (most commonly)
Metal Dryad: Kiron False Garden (if I remember correctly)
Metal Turtle: Aigaion Wilderness
Metal Kiwi: in the earlier dungeons

Apart from Metal Ceratops in the level 500 area, I think it's easier to encounter metal monsters inside the dungeons. Each dungeon/area is assigned a tier in which (only) certain special monsters appear (more commonly). Just use the Attraction Tube until you encounter a metal or gold monster. If you, for example, encounter a Metal Dryad, you'll probably need to switch to a weaker dungeon/area to find a Metal Turtle.

The easiest way to fill the big box/rock bestiary entries is to summon these boxes/rocks (by spending 10 Glowstones) via the Attraction Tubes, though they can also be encountered randomly in dungeons and on the world map (very rarely).