You want to use it to reflect light in a grow room? You're better off painting the walls flat white. It reflects more light, and Mylar gets really dirty and is hard to clean. From what I've heard. I may be wrong, as I've never grown a plant in my life. I killed parsley. How hard is that?
you can get mylar at hydroponic stores, but it is expensive. mylar reflects 98% of light and flat white paint is less than that. a cheaper soulution is getting some emergency blankets, wal-mart sells em cheap. you know those tin foil looking blankets in emergency kits and stuff
what percentage does the blanket put out? btw i saw a vid where a guy used mylar on the walls and had poreous white plastic lining in front of it to increase light, now that worked really well dont know what that stuff was.