Pool filter sand is available wherever you can buy pool supplies. In some areas hardware stores seem to sell it as well, but that's not the case everywhere. In those States of the US that suffer a cold winter during which outdoor pools can not be used, pool filter sand can be hard to come by during that season, but you can stock up in Fall, or you just have to wait until Spring.
People wanting to use this stuff to filter their pool don't want to clean it before use, and they don't want it to cloud their pool either. That's why pool filter sand tends to be very clean right out of the bag. I didn't have to wash mine at all, while for example play sand can be a pest to clean.
Plants grow well in it
I often hear people wonder if life plants need a gravel substrate. In nature, water plants are mostly found on sandy bottoms, and in my tanks plants also seem to grow best in sand. I use Pool filter sand as substrate, and about 1.5 Watt per Gallon of lighting. Fish waste seems to be abundant in my tanks, and that's really all my plants require for vigorous growth.
Valisneria (left) growing vigorously in Pool Filter Sand. Even the Anubias sp (right) is sinking its roots into the sand.
There is only one catch, and that's that pool filter sand is only available in one color. If you want dark substrate, have a look at 3M Color Quartz (now discontinued), Spectraquartz (http://www.spectraquartz.com/), or Estes Ceramaquartz (http://www.permacolorquartz.com/pool.html). Apart from being available in a wide range of different colors, these materials share the main advantages of pool filter sand, in that they have a very similar grain size (see photo above), and are chemically inert. However, they are a lot more expensive. 3M Color Quartz used to be $20 to $40 per 50 pound bag, depending where you bought it. I have not yet had a chance to try Spectraquartz or Ceramaquartz, but 3M Colorquartz needed a very good washing before it could go in a tank. In addition, it can be tough to find a source for these materials in your area.
In summary, as long as you are fine with its light sand color, I believe pool filter sand is the ideal aquarium substrate!