Try Smith and Son Ballast, you'll find they have an ad in Model Railroader. Scenic Express sells some of their stuff, but only some and they charge a little more. You can deal directly with Smith and Son Ballast and get it in quantity. They sell it in small bags or inbilk by the pound. You can also get small bags of the various grades/sizes to see what it looks like in person for a small charge per bag.
We use their unit train coal for the live loads we run and their ballast and cinders too.
They're really great folks to deal with and like their ballast, it is real coal that they crush. I think they have a little rock crushing rig in their garage.
This is their address: Smith and Sons Ballast 13630 Gar Highway Chardon, OH 44024