Go to your local roofing supply company and ask them for Vycor. This is a rubberized roofing underlayement, it is made by Grace Construction Products. It is about 90$ for 200 sq. ft.! This is the price in my area, but even if it were 200$ it is still much cheaper then anything on the net. If your worried about heat reflection, you can glue aluminum foil on top of it with 3M spray adhesive, do it yourself Dynomat eXtreme! I haven't tried the aluminum foil thing yet, but some people I know have and they say it works great!
Oh yeah, there are three grades of this stuff, the only difference is thickness. Just make sure you don't get the granualated stuff. Hope this helps, and saves you some money. You could probably put three layers of this stuff on your whole car for the cost of any Dynamat-like product on the net, but you can buy this stuff almost anywhere. Even some Menards and Home Depots carry it.