I think people use 24 hours veg just because it makes them grow faster. I don't believe changing to 24 is like.. any difference other than speed in growth. About mylar, theyre mostly readily avail online. I can't seem to find that stuff in hydroponics store and I live in a big city.

Edit: I've used 24 hour lighting for my Mother plant, she grew pretty big within a few weeks. But you should atleast allow your lights a 1 hour off time. Not very good to keep lights on for along time even though they're built for that. Just adds more life