where to buy breathable reflective material?

Try http://www.therainshed.com. They carry a lot of varieties of Illuminite fabric, and they tend to give weights fairly accurately, too. They will send you swatches of all their reflective stuff for $1.00. The only drawback with them is you either have to fax or phone your orders–their website is not set up for ordering. Or maybe this isn't the type of reflective you mean…

Joseph,

You’re not the only one looking .

Aluminized Tyvek?

Space blanket? A little noisy but cheap.

Party On,

Newton

Unlike the older foil space blankets, the Heatsheets polyethylene ones are not noisy, and are flexible enough to stretch before breaking. The large model is 5 by 8 feet and weighs about 1.05 ounces per square yard. Cost around $6. Breathable? I had in mind making thousands of little tiny punctures with a meat tenderizer stamp, or you could make a cheap stamp with sharp tacks.

Warmlite sells an aluminized 1.4 oz silnylon, but being sil-coated, is not breathable. Very expensive, around the price of Cuben.

Would still like to know if another oz/sq/yd of Primaloft or the like wouldn't provide more insulative value. But I digress.