Food grade calcium carbonate is often available from businesses that sell wine making supplies. If you check around locally you might be able to find someone that would sell you a 50 lb. bag.
Here is an online source that sells 5 lbs. for $12 and also has free shipping :)
It's a good idea to invest in an inexpensive gram scale to check your measuring spoons and scoops. Many culinary supply stores sell small scales that weigh in grams as well as pounds and ounces. Kitchen measuring spoons are not very accurate for measuring trace minerals where we are often trying to dose in milligrams. Doesn't mean you can't use them (all of my measuring spoons seem to end up at the barn), but weigh out what they hold for each item you are feeding and jot it down for reference. I use a sharpie to write it on the container. It really helps me to remember!
