My suggestion is stop your search and try making your own. I never liked the look of the ore loads that came with ore jennies. I make my own loads using sifted beach wash gathered on the shores of Lake Superior. I also have a supply of chipped stone fragments collected from a well drilling that went through a few 100 ft of solid rock. Really any source of sifted gravel would work.
I model crude ore from northern Minnesota. Crude Iron ore varies in color, size, texture, and iron content therefore, I try to duplicate that using a variety of natural material and colors. I make a base out of styrofoam sized to fit the car. I add relief using modeling plaster, Sculptamold and or Paper Mache. I seal the base with paint before adding the natural stone which is glued in place using methods similar to ballasting the road bed. I sprayed the loads with acrylic paint from Hobby Lobby - American DecoArt Heritage Brick. I then use various colors of chalk and other color washes to break the uniformity of color. It's still an ongoing process as I continue to play with coloration. Kind of like weathering!
Good Luck with your search/or project!
Cheers, Dave