I could never find mylar, but by chance I found like, construction paper coated with mylar at Michaels today. It was 3.49 for a 2 foot by 19 inch sheet and it was worth EVERY penny for a couple of them. Unlike rolled mylar it's stiff so it's easy to install. I put them on the pack of my terrarium (I used to have foil) and WOW! MUCH BETTER! I can grow plants like utrics in places that used to not have enough light!
I'm going to buy a couple more sheets and cover the 2 ends of the terrarium now! WOW way better! It's SO much better than foil and white cardboard!