Easiest/Fastest way to obtain a good quantity of fine Iron powder?
A less laborious path to very fine iron dust (in fact, likely  pyrophoric iron) perhaps follows from first purchasing or preparing iron(II) oxalate (a path is detailed in a reference provided below). Next, thermally decompose the salt and finally, stabilize the nano-iron product with say mineral oil.
The details on the thermal decomposition in an atmosphere of hydrogen is discussed in this paper: Pyrophoric Nanoparticles and Nanoporous Foils for Defense Applications. See also this discussion which includes a suggested preparation for Iron(II) oxalate together with a video here.
Lastly, follow a similar process as is outlined in this US Patent 3,520,676 which claims, to quote:

Pyrophoric powder of metals (such as iron, cobalt, nickel and alloys thereof) is stabilized to prevent its spontaneous ignition. Such stabilization is achieved by Wetting the metal particles with a high-boiling organic liquid (such as an ester of carboxylic acid, mineral oil, silicone oil or fatty acid) and holding the wetted particles in the presence of oxygen until a thin oxide layer is formed on the surfaces thereof.

in order to stabilize the very fine iron product for your intended use.
A particular advantage to this approach is the low cost, and actually, not a great need for a large amount of the nano-particles for your demonstration purposes.