JD i looked in my files for receipts but no luck, sorry. I know I ordered it online from an audio/video place that provided soundproofing supplies.
It was several millimeters thick, dense, came in a big roll (i think it was about 4' wide by maybe 10' long or so), expensive (esp. shipping), HEAVY as heck and a royal pain in the **** to mount on ceiling. Basically it's a 3 person job if you want to do it right (very little cutting, leaving as much continuous roll as possible). Walls were a little easier, but that ceiling....man what a nightmare.
The end result, however, in combination with regular r-19 batts in between joists and 1/2" sheetrock, was that the tv in the room above could be cranked almost full blast and you could barely--and i mean barely--hear it in the room below. This is in a 100 year old Victorian by the way.
edit: click on that link that jpersons provided and look in the "soundproofing materials" section. the stuff i bought was just like this. for that matter, maybe it was it, but from a different vendor (i don't recognize soundproof cow as the place i got it from). as you can see, a 4.5'x20' roll is 180 pounds.
http://www.soundproofcow.com/Quiet-Barrier-HD-Soundproofing-Material-Roll.html
I'd be really interested to know what the rubber sheet product was that you used if you can figure it out. I've getting close to being able to finish off the interior of my barn and I'd like to soundproof it. This sounds like a good option.