In the US, just grab a phone book and start calling places under "Insulation Materials and Supplies" or "Insulation Contractor Supplies"
You can ask for Mineral Wool or Owens Corning 703. Some carry an equivalent to 703/705 by other brands that are cheaper but have identical specs.
My understanding is that the main use for this is for things like boiler insulation and steam pipe/room insulation. With the fire ratings on them, I guess they are used in a bunch of other industrial applications. Typically they will have the paper backed and pipe form too.
Every place I called knew what it was, knew all about it, and gave me prices. Some places sell by the bundle, some will sell by the 2x4 sheet.
When I first did my treatment, Mineral Wool was way cheaper than OC703. But I guess a bunch of it is getting used up since Katrina in New Orleans. So OC is half the price of Mineral Wool.