I ordered mine from Toolstation, but had to call them to do so as couldnt find it on the web and it was only around a tenner for a 10m roll of 225mm wide - i found that 2 rolls did the job.
Really you can only use this on the floor pan and below/behind the rear seat as it will peel off the roof and possibly the doors, but with installing this plus an extra layer of carpet, my car is so much quieter - wow!