How much of the stuff do you need? I'd think that if you're filling up a whole sandbox with it, buying it in bulk might be a lot cheaper than buying individuals bags retail. Shipping should be cheaper because it will come on a pallet on a delivery truck instead of UPS.For example, that sandtastik company takes bulk orders: http://www.sandtastik.com/sandtastik-sand/sandtastik-white-play-sand/128-sandtastik-safe-sand-bulk

Even one of those little turtle-shaped sandboxes will take 150 lbs of sand, so you could probably easily justify a 200 lb order.

However, if I were you I'd just call around to local rock and and sand dealers. The issue isn't with silica in sand (most sand is virtually all silica!), but with extremely fine-grained silica crystals that can be inhaled. In natural sand, this isn't a problem because the same geologic processes that sorted the sand into sand-sized particles washed away all the smaller stuff. It becomes an issue with manufactured sand, where they take larger quartz-containing rocks (quartz is the same thing as silica) and crush them up into sand-sized grains, leaving finer stuff in there too.

But given where we live, I'll bet that practically all the local rock yards and landscape places that sell play-type sand are selling natural sand. Locally-harvested natural bulk sand should be perfectly safe and much cheaper to boot (to say nothing of the environmental costs of hauling sand all the way across the country from Florida where true silica-free carbonate sand comes from).

Now, if you'll indulge me in a slight tangent, I think the safe-sand thing is a bit of a scam. The whole thing got started because in California silica-based sand is required to contain a Prop. 65 warning saying that it contains material known by the State of California to cause cancer. But the risk this refers to is only applies to workers who are frequently exposed to dust produced by sandblasting. There is absolutely zero evidence to suggest there is any risk posed by just playing in the stuff, even if it is the manufactured kind that could contain silica dust. But since the play sand is basically the same stuff as the sandblasting sand, it also has to carry the warning. The kid's play sand with the cancer warning on it elicits images of Irwin Mainway* and so someone came up with the bright idea of selling non-silica based sand that doesn't have to carry the warning. After all, what decent parent would choose the cancer sand? It's a variation of this principle: http://xkcd.com/641/

*http://snltranscripts.jt.org/76/76jconsumerprobe.phtml