Silica Sand

For pure silica sand you will probably have to get it at a sandblasting supplier. It's the only place that I know of. You can use just about any sand as long as is not to sharp edged. I use play sand but many people don't like it for one reason is that it needs a LOT of cleaning. Another alternative that many people use is pool filter sand available at any pool supply store.
 
for the price play sand is worth the extra work, and you can get it at most home repair stores
 
Tried play sand, hated it. Very hard to get clean.
Tried marine sand, liked it. Rinsed once and it cleared up very quickly. A little expensive. Still use it in one of my tanks.
Tried pool filter sand, love it. Rinsed a few times , settled in the tank great and cleared quickly. $8 for a 50# bag. I love it and it looks great too. IMO
 
AquariaCentral.com