hi - fif, first I think you're searching for the wrong thing - you want OC 703, not 403.
When I bought mine, I googled for it in my local area and found it for sale by a local construction supply company. I had to drive 45 minutes to pick it up, but it was easier than shipping.
well, actually now that I remember, what I did was go to Owens-Corning website and searched for a location in my area that sells it. I had to order it special and wait a week before I could go pick it up, but it was pretty painless.
you can also get very similar rockwool - you can usually get that at places like home depot and Lowes.