I purchased a 50 foot roll of duraskrim from www.globalplasticsheeting.com. It is 6 foot wide with a extra inch on each side for nailing. It works perfect for a 4 foot by 1 foot deep bed. A 25 foot roll is $82.50. It is reinforced and can be heat welded.
I am in the process of rebuilding/ upgrading my system from a simple 3 barrel setup. I am using the duraskrim for both the fish tanks and grow beds.