Do some ground work, pick up the phone and call some companies that produce plastic bags, find some plastics experts to talk to, hit the books. Those are some simple research techniques that will garner hard data, and in turn that data will either prove or disprove the claim that it takes around 22 gallons of water to produce a plastic bag. The difference between this and Bigfoot is that plastic bags exist, we know who makes them and we can find out how with relatively little effort.
The burden of proof falls on your shoulders because you are the one who is disputing the claim. Your choice is to produce a verifiable rebuttal or stop saying that there is no "supporting evidence". The evidence is there, the manufactures have it, you just have to find it. Not everything is as simple as conducting a Google search.
I really don't care how much water it takes to make a plastic bag, but I'm really tired of people whining about how the "facts aren't there!" when they search the internet. The internet, Google, FaceSpace, whatever are wonderful tools; but they are not the end all be all. At a certain point you've got to take it upon yourself to get dirty and find out for yourself away from the keyboard. As you've repeatedly said in this thread, think for yourself.