...and it's free shipping in the contiguous USA.
They have 1mm light-duty and 2mm heavy-duty mylar in 50-inch x 25 ft roll, 50 ft roll, and 100 ft roll. The second cheapest mylar I've found online was almost twice as expensive as what these guys charge.
They also have Sun Film Ultra Mylar (97% reflective mylar).
1. Go to http://www.americanag.com/
2. On the lefthand nav bar, type:
in the Search field, and click Search Site.