Most of us are in North America but it could be of some usage to people in Europe. It might also be a viable option to import plastic if you are going to build a greenhouse on a large scale.
Recently, I was visiting the store LEROY MERLIN (something like Home depot) in Warsaw and I came across greenhouse tarp for a very good price.
the price was 37$ for a 6m by 5m tarp (something line 20 feet by 19 feet).
The company that produces the tarp is in Poland and they have several types of greenhouse tarps. see their website http://www.marma.com.pl
If it was easy to carry I would surely take more of it. For that price it is a bargain!