The aluminium (that's how we spell it up here...sometimes) coated, polyester film used in chip bags and many other food packaging is the free equivalent to reflective mylar. Although not designed to have a "mirror finish", it comes close enough for the cost alone.
Think of how much of this stuff you throw away each day simply because it is not recyclable.
We've almost definitely produced enough to cover the planet's surface at least once. If we layed it all shiny side up, would we not be able to reflect the sun's rays back into space and solve Global Warming?
Step 1: What's Good About It?
My favourite source is the plain milk chocolate bar due to it's level of flatness (for reflective surfaces) as well as it's managable size and has nothing at all to do with that sweet, sweet, life giving, coma inducing gift from the God's, chocolate. If larger single pieces are required, chips come in all sizes or tape them together to form a sheet..
Please note that not all packaging is created equal, for varying reasons, and some may suit your need better than others.