Hi all, you can find fine and medium grade ballast and great source of dark media for sides of roads, next to factory buildings ect. at harbor freight for a fraction of the cost anywhere else. Search their site for the ballast 25 lbs Walnut Shell 24 Grit fine Abrasive Media 25 lbs Walnut Shell 12 Grit Course Abrasive Media 25 lb. Black Aluminum Oxide 70 Grit Abrasive Media 1 lb. Spark Plug Cleaner Media I have used these and they will last a very very very long time. I have yet to master coloring the Walnut Shells but am close and will share my results once I get it right. Hope this helps.
I bought a 50lb bag of fine sandblasting sand at Princess Auto, for under $10 .. It worked well for me as the sand was a mix of darker and lighter granules, and I used maybe 5lbs total to ballast my 14x16 layout .. I also used some of it as a scenic base in some areas, and then topped it with normal scenic materials.. It was just the regular sand, not walnut or black oxide .. my era is 1890, so it worked out very well ..
If there's a gravel parking lot nearby, scrape some up and put it in a shaker. A salt shaker is great for HO, but a pepper shaker probably works best for N. Dig deep; you want the powder from the bottom. When shaking it no longer produces ballast, dump the leftover large pieces back on the parking lot and scoop deep again for a refill. You don't get 25 pounds at a time, but the realism is unbeatable, and I defy you to name a cheaper source!