I bought mine here:
CaribSea - Aragamax Sugar Sized Saltwater Sand 30 lb
It's not live, but your LR will seed it and it will become live.
A word to the wise: with white sand (actually any very light sand) your algae growth (you will experience this, we all do), and other "tank trash" will be very noticeable.
Stay away from crushed coral; it will collect detritus and you will then have a nitrate farm. I like my sand; it's a bit of work to keep it clean, but with the right gang of critter, they can help a lot.