I would quote who I believe mentioned this, but, as I'm not exactly sure, maybe he will chime-in here to claim his words?
Anyway, "something like" fiberglass/rockwool/mineral wool are, at least accoustically, the same", or at least substitutable to a great degree, even though the materials may be different. Inotherwords,look through what you CAN find and you'll probably come up with things too similar to worry about their differences(Possibly even structurally "better", for YOUR purposes, than the "standard" Corning 7-0-whatever?).
BTW: At least in the US, and even in my rather small city, there is at least one very large "accoustics" firm! Suppliers(I'm sure mostly) to commercial, industrial and construction firms, but hey, your money spends the same. Actually(With tiny quantities of special plastics, for instance) a call to a friendly salesperson at these kinds of normally large volume wholesalers, with my "story" has resulted in "scraps", etc. that I have been able to get for next to nothing to free("Haul it away!"). Anyway, don't ignore the wholesalers and professional construction suppliers just because you only want a few pieces of this or that.
Also, don't forget that our needs are NOT so special! Corning and other giants would never have developed this stuff "just for us". Accoustics are accoustics and a competent sales person can recommend and sell you just what you need, possibly helping with design and even recommending firms to design/install, if desired, no matter your size or budget - or at least able to recommend a dealer(They are his customers) who can sell you the stuff to do it yourself.
Finally, let's not restrict ourselves to the incredibly limited variety offered(Despite their big buildings) by the Depots and Marts of the world. There are(At least for now) lots of other, better, places to shop...
TG