Perlite question

I know that at some point I saw someone mention (I think it was here at DG, but I can't find it) that you can get perlite much cheaper at home supply places if you search outside the garden section because it's sold for some homebuilding purpose. (Sort of like how epsom is much cheaper if you don't get it at a garden store.) Anyone know what it's called (if under a different name) and what section it's in? Hints appreciated. Thanks.