Keep an eye out for big blocks of alum in the crystal holistic shops...
Some shops sell alum as a "natural" deodorant block as big as your fist.
As others noted there are multiple types of alum out there so if it burns
too much after shaving use as a deodorant.
Since a lot of ladies use these alum blocks and also shave under their
arms it is possible that it is what you want. The package label is always
worth a look.
Your drug store may also be able to order any type of alum you want in bulk.
It is commonly used to demonstrate how to grow crystals... (easier than salt or sugar).
Alum is also used in pickles so it can also be found in many markets.
As others noted there are multiple types of alum out there.....