What I've heard said over and over is that if you're gonna use tin foil, use the dull side, not the shiny one, and keep it SMOOTH. The wrinkles in tin foil can work like a magnifying glass and cause burns.
If you're worried about getting Mylar, just remember, Christmas is coming, and mylar Christmas wrapping is barely more expensive than the regular stuff. Mylar comes in bigger sheets generally and is easier to put up in a growroom, but like I said, I've only used mylar, but I really don't think Tin foil (the dull side) is "bad" for your plants.