Get Mylar. It's works well from what everyone says. Relatively cheap too. You can get a 25' roll for around $15-20. Depending on shipping charges.I had the same idea when I started growing, seems like a great idea but it isnt, the mirror absorb more lights than reflects it, you are much better with painting the wall flat white, when you get a gallon of white paint you can ask them to put more of the white pigment in it, I forgot how its called but that could help reflect more light.