Iron Dragons are found right mixed in with Steel Dragons VIA Dungeon Tele > Brimhaven > All the way to the back past greater demons. They are a good source for Dragon Bones, as well as an occasional visage (rare).
Draconic Visage - 0.12% Chance.
Dragon Platelegs/Skirt - 0.78% Chance.
Dragon Bones - 100% Chance.
Blue Charm - 3.03% Chance.
Green Charm - 33.33% Chance.
Crimson Charm - 14.28% Chance.
Gold Charm - 12.5% Chance.
Note: Also drops lots of misc rune items.