The material I used for the ballast for these tracks is a mixture of sand, gravel, earth and bits of wood. I used a spray bottle with water base urethane finish as the adhesive and binder. The gray ballast in the gondola is made from fine crushed stone which is the same stone used locally for most ballast here in northeast PA. It comes in five gallon bucket sized bags for $5 a bag. This material goes by several names, moon dust, screening, and concrete sand. It is sold in building supply houses, quarries, or other similar places that carry masonry materials. I use a window screen as mentioned above, but for the purpose of making loads, I don't sift out the fine powder. The powder helps cover the edges of the luan I use for the load base. I would screen the fines out for ballast application, if I had a layout.
Whatever material you use, if you're looking for prototypical, find out what material real RR's in your area use, and try to locate that material at the locations mentioned above.
Don
