Where's the best place to get it, is there something I can buy from B&Q that will do the job just as well as a specialist track day exhaust supplier?

i get mine from raceparts uk make sure its for a 4 stroke as a lot are for 2 strokes only and will melt in a 4 stroke

Go for Accoustafil (or whatever it is called). Quieter and more effective than standard glass wadding, so cheaper in the long run. Just don't rely upon the amount the calculator says you need on the manufacturers website as it will order you double what you want!

Got mine from Russbost? on here although via ebay, good service.