Your local co-op or farm supply. If they don't have I'm sure they can order it for you, just ask them. If all else fails call your local agricultural extension. Basic Slag is a great soil additive, If you are growing potatoes it's a must. I use basic slag for my Tomatoes, Hot Peppers & Potatoes.
Other people asked questions on various topics, and are still waiting for answer. Would be great if you can take a sec and answer them