I use Epsom salts if I want magnesium. If I need sulphur I use rock sulphur. I used to top dress pasture with calcium sulphate. In the garden you can just use some broken up gyproc plaster board which contains calcium sulphate. You can also use Sulphate of ammonia as a source of sulphur and nitrogen.
Soil bacteria will turn any of these into a soluble form available to the plant.
