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Have you read 'Master Handbook of Acoustics' by F. Alton Everest?
It's a modern classic. Suggest you use it to determine what you need, and then find a local insulation distribution warehouse.
SONEX acoustic foam and similar foam, even the foam used in the ubiquitous IKEA Puang Chair, are very bad for the sound. It seems like such a good idea, too. Nice charcoal gray or black sculpted foam, just like they have in the control rooms of recording studios. SONEX might be the single worst audio product ever foisted on naive, earnest and gullible audiophiles.