Soundproofing is a huge subject, Mose. I went into it at some length a few years ago when I converted someone's garage into a "neighbour-friendly" drum studio.
Part of it entailed use of dense masonry (not practicable for your needs). I also used a lot of British Gypsum plasterboard, and LaFarge products.
"Soundcheck" board, and "Soundshield" board comes to mind. There are others that, as far as I know, use similar technology to "Quietrock" and another common US product called"Soundbreak"
I can't find either of them over here either.
Others materials to think about are: acoustic rubber, and thin rubber carpet underlays. Before we get lost in the technicalities, have a look here for a good read, if you haven't already done so...
http://www.soundinsulation.co.uk/