This stuff evidently is quite common in Canada but I would like to find it locally to avoid the shipping costs. It is all the same I believe, just different trade names. I am looking in the area of Seacoast NH, Southern Maine (York, Kittery), or North Shore of Boston.
You are looking for a building supply that is more commonly used in commercial construction... You need to look at a commercial supply house that real enterprise scale commercial builders use to obtain building materials as opposed to Home Depot or Lowe's which cater more exclusively towards the residential home supply market and light commercial maintenance...
In New England I would recommend Kamco Supply Corp.
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They will more likely have several different mineral wool products in stock to choose from as opposed to being able to special order one or two products like a local lumber yard... Your local lumber yard is most likely ordering those products from a company like Kamco...
Look in your yellow pages for a commercial drywall supply house. It won't be rare. The bags are fairly large and will probably have much more than you need dependent on what you are doing. A common one here is Thermafiber:
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