the inside of many potato chip bags are mylar, also you can buy super cheep mylar backed wrapping paper at most stores that sell wrapping paper. personally i would rather get a bottle of spray on glue and tack the alumanum foil straight to the walls, this will elliminate all "hotspots" my current growroom that i am building i used an exterior gloss varnish to stick the foil to the box nice and snug, then alumanum tape to seal the corners, after all that i put like 4 coats of varnish on top of the foil to make it good and durable. most grow shops sell mylar pretty cheep by the yard, but screw it, i have always liked foil for small spaces...