Many years ago (25+) I bought a MT Z-scale starter set that included a loop of Marklin track. I immediately noticed that normal N-scale ballst would be too course. Then a few months later, while visiting a Michael's craft store, I discovered a jar of black Activa brand Scenic Sand. It's a very fine grain. You can find it at Amazon in black, white, and different tones of gray and brown. Mixing some of these, in various ratios, might produce a decent enough looking ballast. 1lb bags run from around $8.00 to $15.00.
There are also other brands available in packs of 1.25oz bottles, in multiple colors (Including the white, black, grays & browns), for little money. Though most of the colors wouldn't be very usefull for MRR purposes. And I can't speak to the grain size of them. Just do an Amazon search for 'scenic sand'.
