Mylar is the best known reflective material for reflecting "photosynthesis-ready" light. Ergo, your best option...

Don't be discouraged though, you can use FLAT white paint to achieve the next best reflective element.

What you shouldn't use is mirrors, tinfoil or other aluminum objects (big no no), any type of glossy paint, and last but not least no shiny metal plating...These types of materials/objects DO reflect light, and even some usable light but they also reflect A LOT of heat and can actually cause the light to form condensed and focused light waves (like a magnifying glass and an ant) causing "hot spots" which will burn your plant/s...

Hope that helps you make some decisions...