Tyvek (the stuff they wrap houses with before they add the siding) is a gas permeable membrane that is pretty much available anywhere. I've only seen it sold in the big rolls, but if there's any new houses being built around, your sure to find scraps of it.
I remember I worked for a place that sold raincoats made out of Tyvek. The company gave us a display that was a clear cylinder with a sheet of it in the middle dividing it in two. Water was poured into the top chamber and an air pump was connected to the bottom chamber which forced air through the Tyvek and up through the water. Even while the pump was off there was never any water in the bottom chamber.