Beach sand?

I'm pretty lucky living in FL. I hit the beach or sand pits and colect the stuff that has been blown around. It drifts up in areas and is extremely fine. I've even mixed it with betonite and been able to sand cast brass with it and have gotten some excellent results. Because its been blown by the wind, there is almost no organic material in it.

The fine sand at DIY centers will work dependent on scale. You'll have to judge that for yourself.

In a pinch for really fine sand I've seen the ground laid out with celuclay then coated with a mix of white glue and water. Baking soda was sprinkled on top and once dried spray painted with the appropriate colored paint through an airbrush. Looked pretty good. The guy got the idea form the guys building snow dioramas.

Would silica sand - used in sand blasting - be too course?

It's available in 25 lb. bags at Lowes, Home Depot, etc. and is inexpensive. One bag would last a life time.

Go to this web site

http://www.greatmodels.com/

Select search and type in ballast. Woodland Secincs has it and their stuff is great.

Good luck,
Richard

Beach sand would scale out too big! It is also contaminated with salt! In some countries, it is illegal to take sand from a beach!
It is better to search for a finer product. For 1/35 scale dioramas, I use Silver Sand which was bought in a 5 kilo bag from a DIY shop. It is very fine and is more the right scale for dioramas. I also use it to mix with plaster when making earth - it gives a much rougher look which, after painting, looks much more like earth than only plaster.
It can also be used to make the 'white water' wake of a vessel in motion - use neat plaster for the sea surface and a plaster-and-sand mix for the wake. It makes the white water look churned up with air bubbles.
Hope these comments help.

This is just one solution. Hope it helps
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=5902228704&category=2588