I do Scholastic every month...can't beat books for $1. No minimum orders and lots of choices plus you earn reward points rediculously quickly that you can use for more books or classroom equipment.
Our library has a free book room where people can dump unwanted books so I look there often. Also parents donate unwanted books to me too.
I also do the Kohls books, but I have to say I do buy the stuffed animals for my dog (LOL!!
) because they make super dog toys and for $5 you can't go wrong.