Google is your friend.....only thing about mylar is it is really cheap. It cost more to ship it than the actual product does, kinda like rockwool. Rockwool cost about $10 - $12 to buy and $15 to ship it....in other words buy the cheap stuff(soil, mylar, hydroton, rockwool) at the local hydro shop, and buy the big stiff ( lights, ballast, reflectors, Co2 generators) online.
Unless of course you are buying a bunch of stuff at once ( like a whole setup) then you can throw in the cheap stuff in with the shpping. Might even ask for a free roll of mylar or bag of hydroton, they usually either laugh and play it off as a joke, or they actually throw it in there for ya.....worst thing the can do is tell you no.