I got Mt. Hardware's "Do It" brand, and it contains KCl (potassium chloride), NaCl (sodium chloride), NH2CONH2 (ammonium something), and C7O6H14 (can't remember my chemistry well enough to know what that is). I got several small (20-lb. bags), as I can't lift a 50-lb. container.
As for storing it, I just leave it in the bag, and use a coffee can to spread it. It doesn't clump just sitting there.