I just found this page:
http://www.transitchicago.com/assets...es_outlets.pdf
which lists all the retail operations that sell passes. Jewel/Dominicks/CVS and a ton of currency exchanges, among other places.
I know you can by one at ORD, and the CTA has a big office at the State/Lake stop. The 3 days don't start until your first use, so you can buy them ahead of time.