Please see this link and you can also join the group if you wish.
Much of what you see being used commercially is the the Duraskrim 20WW LDPE Polyethylene liner as in the discussion that Harold pointed out. We are a distributor for is it was hard for folks to buy less than a 100' role and people often need small quantities for small systems. We usually keep a decent amount in stock. Please see http://www.greenacreorganics.biz/index.php/what-we-offer/system-sales/ for more pricing information and here is a data sheet for the Raven product. http://www.ravenefd.com/pdf/DSR20WW.pdf Thanks!
Thanks for the info, I will be ordering some soon.