I'm about to start the production of my first grow box. I would like to use mylar, as it's known to be the best material to use to line your box with, but I can't even begin to find a place that carries it, or knows where to find it. But I've seen in a few threads where people have used the reflectors meant to be placed in a car windshield to reflect away sunlight. Now my question is; Do these actually do a fairly decent job at reflecting light? And if so, is it comparable to flat white paint, which I believe reflects around 85% of light.

Depends on the brand, You can find mylar on ebay for decent prices. I've heard of people buying wrapping paper (the kind for Christmas presents). The back of the wrapping paper is silver and reflective.
A cheap and available alternative.

I ended up going with the ebay idea, haha. I had a friend order it for me so there's no connection, just in case.

ohh wow your freind knowing... FAIL haha jk

__________________
good thing no one took this yet

oh noes cant let them find out your ordering mylar...... dollar store metallic gift wrap for the win.