on the wiki it say the following to location of ferrous ..
Ferrous Ore is a primary source of nickel as well as a source of Platinum Dust, Pulverized Shiny Dust and Iridium Ore. It can be smelted into Nickel Ingots or macerated into Nickel Dust. It is most commonly found on layers 11-14, and more rarely on layers 15-17.
Ferrous Ore can be smelted into a single Ferrous Ingot. Pulverizing Ferrous Ore in a Pulverizer produces two Pulverized Ferrous Dust and has a 5% chance to make Pulverized Shiny Dust and a 2.5% chance to make two Pulverized Shiny Dust.
Also I cant remember for sure but if you get some Nickel dust as byproducts of other processes you can smelt it to make the ingots then pulverize the ingots to get back to ferrous dust
It can be found in most types of forest biomes: Forest, Plains, Rainforest, Autumn Woods, Birch Forest, Forested Hills, Forested Island, Green Hills, Meadow, Redwood Forest, Redwood Lush, Woodlands, and so forth. In the Twilight Forest, Bauxite can also be found in Dense Twilight Forest, Dark Forest and Enchanted Forest.
(or it can be found at my base - take what u need