The plastic came from a Master Gardener co-op we had with a wholesale supplier.
It was 6 Mil Greenhouse Film 24' X100.' I bought a whole roll, but we would sometimes divide the roll into three parts. A roll cost about $90. at the time. There was also a 32' X 100' roll available for slightly more. The plastic that the big box stores carry is not very good for long term use.
Edited to add: I just spoke to Greenman, who was one of the originators of this greenhouse. He said the 4 mil is fine if it's pulled tight, or only used for one season. The wind whipping the plastic will make it crack if it's not tightly secured. Greenman said to use the 6 mil (which is what DH said we used, but I read the co-op list and saw 4 mil).
