I get frequently asked about where to find the "real" stuff for acoustic treatment as opposed to the ineffective myth stuff like egg crate foam and such.
I bought bought my 4lb Roxul by looking in the phone book for "industrial insulation" suppliers in the yellow pages. A local place had it and I bought a truckload. This was MUCH cheaper than buying acoustical product websites online.
If you have a choice between Roxul or Owens Corning 703, I'd highly recommend you go with 703 just because you can actually touch the stuff without wishing you were dead. I can't say the same about Roxul. The stuff DEFINITELY works, but I wrap myself up like I'm going into Nagasaki back in '45 before touching the stuff.
I also highly recommend the 4" stuff. 2" ain't bad, but it doesn't absorb nearly as low as the 4" variety and you really can't get enough absorption in the lower frequencies.