How much steel would you need? Going by dip pens, let's see.
You get about 1.6 million steel pens from 120 tons of steel with 19th-century manufacturing techniques. That's a pretty consistent number from a couple of sources.
In 1864, Birmingham alone produced 98,000 gross steel pens a week. That's 14,112,000 pens a week, or 733,824,000 a year. That's about 460 tons of steel a year to produce 14 million pens. Now, these pens were in different sizes, some smaller, some larger than fountain pen nibs, and they were most definitely thinner, but then we're wanting to make flexible nibs.
I doubt anyone would sell 14 million flexible nibs. That's not a lot of steel, but then I have found no evidence that they used any exotic alloy of steel. I bet, pretty much any good, thin, spring steel that could be cold-rolled would work. Actually, let me contact the pen museum in Birmingham where you can still make your own pen using the old machines, and see what kind of steel they use, and if they have more detailed information on the steel used at the height of the steel pen industry, 1880-1920.
I'll let y'all know what I find out.