Tantor is running a 50% off sale on all their audiobooks for the month of June.
www.tantor.com
All formats (including MP3 downloads), and free shipping with $25 order.
I've picked up a couple of audiobooks here: I think their prices are generally good, and didn't have any problems finding interesting stuff.
As always with audiobooks, I'd suggest listening to a sample-- the narrator's style can make or break an audiobook for me.
KentE