I wouldn't trust sand to be clean from a beach/lake. I'd be worried about pollutants/oil/insecticides and everything else that gets dumped in waterways. If you do choose to use it, I'd soak it in bleach, followed by dechlorination and lots and lots of rinsing.
You can use play sand or sandblasting sand from Home Depot/Lowes, and it will only cost a couple of bucks. All you need to do is rinse before use. Sand is pretty commonly used in fish tanks. Both of the above kinds are fish safe.