I dunno about the ability to meet your requirements, but there are iron powders available for various uses, from toys (Etch-A-Sketch), to industrial uses.. electromagnetic clutches/brakes for machines. These examples are moderately coarse powders.
Very fine iron/steel particles are a byproduct of etching/cleaning or pickling steel products in manufacturing. The steel parts are pickled in various acids, and the particles which are suspended in the liquids are then filtered to remove the particles from the acids. The result is a mud/clay-like material that is used to manufacture other products, including iron composition cores for certain magnetic properties.
I don't have any sources to recommend.