Time for shopping most likely.
OTOH, you may also try acetic acid - glacial acid (that is, one without water, that can be easily bought) behaves similarly:
[CH3COO-][CH3COOH2+]=10-15.2
Of all substances listed so far this is probably easiest to find, and most safe.
Other substances to consider - ethanol:
[C2H5O-][C2H5OH2+]=10-18.9
and methanol:
[CH3O-][CH3OH2+]=10-16.7
Both should be easy to find in anhydrous form.
Note, that even if these substances are bought or prepared anhdyrous, just opening the jar/bottle means contaminating them with water.