Dan_Musicant writes:

The pool supply places say they don't carry it ...

That's odd. It is a necessary ingredient to protect any plaster pool
surface from soft-water etching. Maybe the pool supply places are in the
replastering business, too.

Another source is concrete suppliers, where it is used as an accelerant.

I would also ask at an Agway feed store type place.

Or just the 42 oz DampRid refill at Home Depot.