I buy mine in 100 lb bags from a local stucco/drywall place that's in the industrial section of town. They also had hydrostone as well. It's been a long time since I picked some up, but I believe it was 30 bucks or so a bag (Central CA prices).
Check your building supply places instead of the big box stores. Unless your in a pretty rural area I bet it can be found locally. I can't imagine what shipping would run on a bag.