School Specialty Publishing has tons of workbooks and study material on sale --
I even got a full summer study course for one of my kids for only $1.80! If you spend more than $35 in material, you can get free shipping by putting in the following code: HPFS3078
Check out http://www.schoolspecialtypublishing.com and see over 600 items in the clearance section. Flash cards are only $0.60!