Found 1 only on levels 21F - 25F. Don't know whether there is any relation but I actually found 1 Ferromin L prior to this on the same run. Should be random though.
Found 1 on levels 1F-10F, but no Ferromin L. Does seem to be random.
Can confirm: I also found one on one of the first floors. I think it was 3F.
Very rare drop, players will likely have collected the other two ingredients for Invincible Armor long before gathering the 3 required for fusion. Collect 6 if there is a wish to equip Louie and Arma with the best heavy armor available.