Typically the most difficult basic material to obtain for electrostatic cell fabrication is suitable material for the diaphragm. This task is even more difficult if you intend to tension the diaphragm by heat shrinking, as I do with my ESLs; this is because the heat-shrinking properties of plastic films vary substantially, even among the films made by DuPont.
It is generally accepted wisdom that DuPont Mylar or equivalent is best for ESL fabrication. During the 90's Roger Sanders was kind enough to supply builders with Mylar Type S film, which I used to fabricate my first cells. I think it was 12&mu thickness. According to the data sheet for 12&mu Mylar Type S, it will heat shrink up to 3.5% if heated at 190° C. I found that this film worked beautifully for ESLs.
However, recently I have not been able to find more of the Mylar Type S film. Instead I tried some Mylar Type C film that I bought from a seller on Ebay Stores. The film was thinner than 12&mu and, much to my disappointment, I found that it did not adequately heat shrink. I later found a data sheet for Mylar Type C and discovered that not only does it not heat shrink as much as Mylar Type S, but it also does not heat shrink uniformly; it will shrink up to 2.1% in one direction, but only by 1% in the orthogonal direction. Type S, on the other hand, will shrink uniformly, at least for the 12&mu thickness.
I talked with someone at DuPont Teijin Films and I was told that Mylar Type S film is no longer manufactured. However, there is another film called Melinex Type S that is supposed to have equivalent properties and is still available. Unfortunately however, the minimum quantity that DuPont requires for direct sales is prohibitive to say the least - they require an order of at least 5000 pounds!
Luckily there are other ways to buy this film - DuPont has a network of distributors that will sell their film in smaller quantities. But there are still some catches. You have to find a distributor that happens to carry the film that you want, and even if you do you will probably have to order much more than you would prefer. The distributor I had the best luck with was Plastic Suppliers, Inc. , but I still had to buy 20,000 ft. of the film. The film that I bought was 18 inches wide and 12&mu thick; if this size is adequate for your ESL project, I would be glad to provide moderate amounts of the film to builders; please email me at: firstname.lastname@example.org.
One important decision a builder must make is what diaphragm thickness to use. In reading the literature about ESLs, you get the impression that "thinner is better"; after all, one of the virtues of ESLs is that the diaphragm is nearly massless when compared to conventional speakers, right? For example, ER Audio offers some 3.5&mu Mylar Type C film that is described as the best performing material for ESLs. Well, I think that depends. In the first place, thinner material is more fragile and more difficult to work with, so I think you should have a good reason to use it. And if you are going to drive your electrostatics in the usual way - namely with a constant voltage source amplifier - I would argue that thicker 12&mu film is actually preferable for the following reasons.
Peter Walker of Quad fame has shown in a JAES paper  that an ideal, massless electrostatic should actually be driven by a constant current source, not a constant voltage source. Let's say that you have such a speaker, but you drive it with a constant voltage source instead - what will happen? Basically the impedance of an ESL looks like that of a capacitor, which means that it is inversely proportional to frequency. Therefore if you drive it with a constant voltage, the current will increase with frequency. So one can conclude that driving an ideal ESL with a voltage source will yield a response that rises with frequency, rather than a flat response.
Now, I drive my electrostatics with a constant voltage audio
amplifier through a step-up transformer, and I do not observe
the rising high end that is predicted by Walker's analysis.
I think this is because my speakers do not fulfill the
assumptions that are made in his analysis, particularly
the assumption that the diaphragm is so light and unconstrained
that it does not disturb the surrounding air. In fact, the
far-field response that I measure from my ESLs is nearly
flat, at least in the optimum listening position right
on axis; if I were to use a thinner diaphragm material, I
suspect that I would observe a rising high end (however I
must admit that I am speculating about this, since I have
never tried it). But the point is, if you are going to
drive your speakers with a constant voltage amplifier,
I think that the thicker 12&mu material may very well be
the best choice.