Weathering powder is ground up pigment in dry form. Some folks grind up pastels or graphite, but there are places you can go online and order them.
They aren't cheap, maybe $5.00 to $6.00 for a screw top jar about the size of a GW paint pot. They should last you practically forever as long as you aren't doing a 1000 models or a basement sized train layout.
They are also available in several colors. Black, rust, fertile soil, parched soil, dust, green patina, etc.
I got away from pristine showroom floor armies a while back. I like my armies to look battle worn at least, Valley Forge in extreme cases. They have a lot more character that way. Powders are easy, quick and simple to clean up, just brush off excess before you clearcoat.