Hey guys this is a tip I spread around the 501st a while back but I don’t know if its made its way here or not. If your looking for a good cheap supply of fullers earth just stop by a local pet shop and pick up a tub of chinchilla dust bath. I picked up a can years ago and im still using it.
I was only able to find it in gray but it was a 2lb can for like $5.
So spread the word fellas.
Not really used around here, most of the props are black and weathering mainly consists of metal scratches. Good tip though. 
I got mine off the interwebs from some prop company that specializes in Indiana jones stuff. Forgot the name of it
Boom goes the dynamite. Found it
http://www.toddscostumes.com/costume-accessories/movie-dirt-for-movie-costume/
I know most of the weathering is done with paint but I have seen a few of the guys using the fillers earth to give their packs that natural dusty look.