I read through the Wiki and it stated "...where wide bandwidth is not essential use dielectric coatings, which can achieve reflectivities as high as 99.999% over a narrow range of wavelengths." So it sounds that the quality of mirror is what is important. Your standard Walmart might not carry the kind of quality mirror you need or deliver the wavelength your plants can use. But, I think what many here are missing is the ability to think really about what is happening to the light with a mirror. Light reflects from my body at a great number of angles, but only one "line of light" is able to penetrate the outer glass coating. The glass (silicon) acts kinda like a crystal, focusing the spread of light into a parallel band. Hence, red is reflected as red, at an angle opposite to its arrival, and blue as blue. As a mental experiment imaging a plastic tube a few inches wide. Golfballs roll down the tube constantly, and bounce off the other side and start traveling back up the tube they just came through. There's only so much room so a bunch of the balls collide. These collisions reduce the number of balls that can complete a trip. There would be many tubes for the different wavelengths of light, or colored golfballs. This keeps the image from blurring. My guess is that if you could peel the coating from the back of a mirror off, or go to a glass shop, I'm sure they sell that stuff in rolls for mirror makers, that would probably be the absolutely most efficient reflector. As I understand it, what you're looking for is something that doesn't get hot while having as few layers between the light and the surface. Mylar is an effective reflector of the right wavelengths, which is what makes it ideal for growing. White paint is probably the best reflector. Since white is all the wavelengths of color smashed together it is probably (but don't quote me on this) more natural for the plants than the specific angular wavelengths you would get with a mirror. Maybe those convex mirrors that are used on single lane roads around corners would act as a good reflecting surface since its weatherized and all.
I might be a bit late on the conversation, but heres a nice graph I found in a guide. From "Marijuana Horticulture The Indoor/Outdoor Medical Grower's Bible"