The Sonopan panels can be sourced through just about any building supply store. Talk to someone in their contractor sales department and show them some info. They'll be able to get it for you.... It does work very well. We've been using it in apartment building construction for years as part of a soundproofing system that includes much of what flashinstinct mentioned.

If you're really concerned with soundproofing any area (like the theatre), you might also want to look into Mass Loaded Vinyl (MLV). It's heavy and fairly expensive, but it works better then anything else I've tried.