I have collected sand from a 5m deep rock pool in Seaview, PE. It is a long gulley with pristine sand at the bottom and could only gain access to it at a very low tide. That sand seemed even finer than beach sand, perhaps it was being swirled around at the bottom of the gulley and in the process being eroded.
What about collecting beach sand off the top of high sand dunes. The wind blows the finest sand along the surface (any heavier stuff like metal pieces would be heavier and therefore not present) of the dunes and perhaps that could be collected by skimming off the surface layer of sand from the dunes. This sand would be dead but could be seeded with live sand once in the aquarium. What do you guys think?