Try to find dolomite. It still buffers, however not as well as aragonite. I called my LFS
and got some in 2 days when I couldnt find aragonite. I was charged $14 for a 50lb bag. That is a far cry from $24.99 for 20lbs.
Aragonite - calcium carbonate
dolomite - calcium Magnesium carbonate
I was told by a local sand and gravel place that is most commonly used in swimming pool filters. Give some pool supply places a call.
Try to get the dolomite before you fo
with the silica. If you waited this long another day of searching won't hurt.
Hope this helps.