Most if not all States require various permits to mine river sand, gravel, etc. Contact the appropriate state environmental, water and mining agency for a listing or an inspector knowledgeable about ongoing activities. It is not unusual to have to pay a premium for some types of river sand. Supplies of sand can be costly to transport distances, so best to find one locally. You might also contract local concrete companies concerning various sources of sand.

If you want it severely nutrient-impoverished, you might want to acid wash whatever product you buy. It may not matter with a pure sand. I don't know what quantity you're thinking about. Or what system the sand is supposed to mimic.

The biggest use of sand presently is for fracking for enhanced oil and gas capture. The biggest production area is near the Wisconsin River in Wisconsin in the so called sand hill country. This sand is mostly extracted and shipped to North Dakota where it is employed in the Bakken Oil and Gas field.

there are many gravel pits in the Connecticut River valley of Massachusetts removing all those glacially sorted sands from the Glacial lake Hitchcock formations.

If you want severly nutrient impoverished sand, plz collect locally and then wash it 2-3 three time