I got mine at tulumba.com for $2.99. They also have good prices on arko stuff if you're interested (I tried them, but am not a fan of the scents).
Barber's Alum (Barber's Alum): Tulumba.com
When I ordered I googled around and found a code for free ups shipping. I'm not sure if there are any still floating around, but it might be worth the time to use your web-fu.