You really want to use EPDM 45 mil liner for building any pond or tank. It's the Firestone inner tube material that is guaranteed for 25 years. Anything else is a waste of time and damages easy. You pay for what you get. Good Luck.
I found this same website when I asked people on here about a build I want to do. Someone recommend the EPDM liner and also said I could use billboard tarps as well. Mine would be housing turtles but still not sure what to do.