The Grand Tourney housing gauntlet, currently available online, definitely does drop brass and steel, plus pet snacks. The gauntlet is challenging to solo but fairly easy if you take at least one other friend in with you.
Star Jewel Blossoms are the very best gardening source for brass and steel. Unfortunately, the plants themselves are an extremely rare prize that come from the Farley's gardening pack and/or other jewel blossom plants. I got mine by starting with two Circle jewel blossoms and after about 15 harvests, one of them dropped a Star blossom. During the last double gardening event, I doubled that single into two, then doubled it again - so now I have four. With all their "likes" on the jewel blossoms go through their whole life cycle within three days. Super fast!
As for bosses that drop steel, there are far too many to name. Starting at Celestia and beyond, both brass and steel are literally dropped everywhere, by practically everyone. Just ... rarely.
All you can do is keep questing, and pay attention during mob and boss fights. When you notice that you got steel as a prize, note that down.
(As for me, I've noted that the Gladiator Dimachaerus in Mt Olympus drops both brass and steel, plus the coveted Potent Trap treasure card. He's available to fight if you're level 30 or higher, but TOUGH to solo. Take friends. He's hidden behind the 'chess game,' across from Hephaestus's Hall.)