In the past, Ive found and bought alum in the spices section.
However, it is an essential in making many types of pickles, so the
canning area is a second alternative. Also, I believe alum can be
used as an astringent, so ask a real pharmacist [as apposed to a
pill counter at mass merchants], though it may have to be special
Note: I asked at a grocery store and they said no, but it was on the
shelf anyway, only the spice person knew of it The general manager
nor office clerks knew, the spice person didn't know why, but knew
it sold lot, especially in summer and fall. :<)
Good luck, and sharp keep a sharp look out. . .