Mylar is good, but it has downfalls. It tears easily. I recommend solid walls. Fans alone can tear it up if the wind moves it too much and it starts flapping.
Just to let you know.
Not sure if you would find it at a typical gardening center. Most hydro stores, yes. For first time setups, in order to keep costs down, I recommend flat white painted walls. That is probably the cheapest....and still works great.
White poly is also good. More forgiving than Mylar and cheaper. There are a few other types of reflective materials that work well and are tougher than Mylar. Check out a product called Foylon. Much like Mylar, close in reflectivity, but MUCH more durable.
Don't get me wrong Mylar is great, and adds a trippy effect to your room, but I just want you to know the possible disadvantages of Mylar before spending a good chunk of money on it.....it ain't cheap. It has also been reported to create hot spots on plants, which might lead to leaf or bud burn damage.
Never saw this myself, but it is claimed widely enough to be a valid possible disadvantage.
Another problem with Mylar is that is can literally get scratched up if you wipe the walls when you clean your room.
I am in no way trying to dissuade you, as it is an excellent product....just finicky and pricy.
In fact, here is a guide that I used once before from another site.
http://www.420magazine.com/forums/how-grow-marijuana/71077-what-reflective-materials-can-i-use-my-grow-room.html
FORGOT ONE REALLY BAD THING ABOUT MYLAR.....NOISY AS HELL WHEN MOVING AIR FROM YOUR FANS HITS IT.
