The Insulation Debate
Extensive marketing shows that specialty insulation products isolate sound much better than standard fiberglass batts. The problem is extensive sound testing shows that specialty insulation isolates sound just as well as standard fiberglass batts. The use of insulation in a sound rated assembly is crucial, however, the insulation type generally is not. As long as the insulation is light, fluffy, and sized to fit the cavity, then it will provide the proper amount of absorption.
Specialty insulation manufacturers will dispute this of course and often provide an STC rating with the specialty insulation used in a high STC assembly. What they conveniently leave out, however, is an STC rating comparing the performance of standard fiberglass in that same assembly. The absorption rate will also be touted for superior performance to fiberglass, but labs conduct absorption tests with the insulation exposed. Once rigid mass covers the insulation (as it would in a wall or ceiling), the differences in absorption coefficients are, more or less, equalized.
Specialty insulation does have a place in construction for other reasons, increased energy efficiency, fire rating, easier installation, and fewer chemicals - depending on the insulation. However, for sound isolation, save your money and buy the cheap stuff.
BUYING INSULATION ONLINE
Rarely is this a good idea or a worthy investment. The big manufacturers have an extensive network of local distributors for insulation products that should be able to get you the product you need for significantly less than what you would find online. Even if you do not find what you need at Home Depot or Lowe's, ask them to order it in for you through their contractor services.