Steel Ores can be bought from other players or found from monsters. You can also obtain Steel Ores from the Porter's Challenge (Level 50) quest.
The list of monsters that drop Steel Ores is as follows:
Blue Snail, Dark Stump, Stirge, Desert Rabbit (F), Urban Fungus, Dark Axe Stump, Seacle, Rocky Mask, Green Trixter, Bubble Fish, Brown Teddy, Boomer, Drum Bunny, Meercat, Cloud Fox, Fire Boar, Mask Fish, Nospeed, Leatty, Jr. Grupin, Star Pixie, Jr. Boogie, Cold Eye, Ratz, Tweeter, Barnard Gray, Triple Rumo, Chipmunk, Deo, Extra B, Ginseng Jar, Jr. Cerebes, Wraith, Block Golem, Firebomb, Grupin, Hogul, Tippo Blue, Hector, Minor Zombie, Pepe, Morphed Blin, Buffy, Klock, Rash, Extra D, Yeti & Pepe, Lycanthrope, Wolf Spider, Dark Yeti & Pepe, Red Kentaurus, Bone Fish, Red Dragon Turtle, Spirit Viking, Grim Phantom Watch, Red Wyvern, Dark Cornian, Skelegon, Gate Keeper