Try Quarter Priced books! My local Half-Price Books, which I never go to, had an awesome selection of Osprey Books for sale. All of them $5, including the thicker "Essential Histories" series. I got several including the on topics of Inch'on, Celtic Forts, and Pirate Ships. I think I got about 7 all together. I didn't even get all that they had. They had 5+ copies of several of the titles, and some more unique titles that didn't interest me much.
Anyway, if you got a Half-Price books around you, probably worth checking out if you're into Osprey.