Stone "dust" might be a ballast alternative worth considering if you can't otherwise get what you want. It can be found at home improvement stores in 40lb. bags and at most places that sell crushed stone, as it is a byproduct of the stone crushing process. The coarseness of it will vary, and most of it contains different sizes of stones starting at powder on up to about 1/8" and sizes in between. So, if you wanted to go this route, you would probably want to use one or more different size mesh screens to separate out the dust and/or the larger stones. If you need a lot of ballast, this would be less expensive than buying ready made ballast.
I've also seen mention from others on this forum that they collect the stone dust that accumulates in low spots of paved parking lots for their ballast.