I agree with Simon. It is easy to get for the film manufacturing companies from the film base manufacturers. However, since the film base manufacturers like to sell in huge quantity, it is hard for me (or any individual) to buy in small quantities at reasonable price.
A friend asked for a sample from one company and got a roll that weighed close to 1 ton delivered to him as a free sample and he has generously given me a bit of that for my work. Otherwise, I would have none at all, as I don't need a 1 ton roll, and I certainly cant afford to buy a 5000 ft 42" or 72" roll.
PE