You will not find Southdown (or aragonite based) sand at any Home Depot's around here in the PNW. I've tried already.... I know, however, that they will order full or half pallets from the east side U.S..
I got a bag from someone who was in on a pallet order and ended up not using his. Truly lucky find if you can find a Southdown bag unopened here.
Tank take downs is another starting place. I have preference on Southdown for its non-uniform particle size, but still function for a DSB.
- Elmo