Powdered sulfur is sold in chemical supply stores. It's usually labeled "sulfur", "sulfate" or "S". The "powder" part of the label means it's fine, and not a coarse rock powder. I've seen it in a couple of different grades, but I'd suggest the cheapest one that still gets the job done.
You can use elemental sulfur (sulfuric acid) or sodium sulfate. These are sold in garden centers as "plant food", or as "plant food pellets" (I have not seen them for sale under any other name). They are very cheap and easy to use.