When I made my substrate, I just went to the Home Depot and went through their soil section one brand at a time. I either looked for a torn bag or tore one a little myself and dug in. If it looks like mulch, potting soil (has perlite/fillers or fert spheres), compost or if a handful feels warm or wet, try something else.
Here's what I bought. It was dirt cheap (yuk yuk). I think $2-3/40lb. bag.
A handful before any screening
Of course, not pre-treating it or anything I had a huge green water problem, but that eventually subsided. That's another reason I'm so keen to try the mineralized method on my rescape.
A few months later...