Where to buy Poly?

Where can I buy some of the Poly (white/black plastic)? I looked at home depot and they dont have anything even close that I could use. I don't want to go through the trouble of building and painting walls.

I was also considering Mylar but wanted to go with Poly because it's cheaper. But i can't find mylar at stores either.
Thanks.

Most all grow shops have it, check online or around town

hmmm gave me an idea for my expansion... + rep!

most of the time that stuff is more costly then mylar, try doing a search for panda film
I paid almost 100 for a roll 10x100
another idea for you, my brother came up with this woven paper that is used for fire protection over insulation in construction not sure what its called
but its white on one side and shinny like mylar on the other, if you could find that stuff maybe its cheaper then panda film
Good luck