Iowa State University Extension has a great resource for ag teachers to utilize. I use this site for readings, student activities and calculators. The readings available here rival any textbook I've ever tried.
Home Page for Ag Decision Maker: http://www.extension.iastate.edu/agdm/
The site administrator asked me to post this for you all to be able to use. Go to the link below to get the teacher keys (answer sheets for activities) and use this username and password: Login: agdmteacher Password: takeys