Check your local Home Depot, Lowe's or similar store. You can usually buy it there either by them cutting it off of a large roll and paying per linear foot, or you may have to purchase a pre-cut piece that is packaged for a set size. What are you using it for? You may be able to get away with using some heavier vapor barier plastic depending on your application.
I did a water feature, and went to a pond message board and asked for some scraps. Person sent it to me for the price of shipping... a nice 3 foot by 3 foot section.
I made one water feature out of it, and don't know if I will do it again. If I do do it again, I will do it outside on a hot day in the sun, as the liner becomes MUCH easier to work with. I worked on mine in the basement, and it was a PITA (very hard to control) and I'm still not happy with it (though it works). I tossed the scrap on my back deck, and when I got around to cleaning it up one hot saturday, the stuff in the sun was a maleable as saran wrap.
Moe, I have used a 3mm "contractor" garbage bag, then lined it with aquarium silicone (spread it thinly with a throw away taping knife) then imbedded fine gravel into the still wet silicone. When the silicone dries you can shake off the extra gravel. I used this for streambeds in several tanks. It was pretty flexible and you could cut it with some old scissors. When I broke the tanks down a few years later the plastic had started to degrade but the silicone was fine.