First of all, I suppose that everybody knows that the fixer bath is constituted with a chemical capable of dissolving silver halides without damaging the silver itself that constitutes the image. Normally there are two substances that are mostly used, sodium thiosulfate or ammonium thiosulfate also known as hyposulfites.
Searching the internet you will find references to other substances that may be used as fixer but nobody gives you results of their own experiences. Well, guided by the references I could find, two other substances went under my experimental tests: ammonia and common salt, sodium cloride. And I can announce you that both this substances really work. So, from now, you are not limited to the special substance used in photography, you may replace them with either ammonia (ammonium hydroxide) or common kitchen salt. But...
Ammonia is available at drugstores, I have no difficulty in purchasing ammonia at a big drugstore called AKI (Portugal), it works fast but the fumes are terrible. But in former times I used to make heliocopies of drawings and the developer was also ammonia and we were used to it and I am still alive. So, if you can stand smelling a little ammonia, open the window and use it as fixer, quickly, and put back in the bottle.
Common salt is also a fixer, it dissolves silver halide. As first use it very concentrated, I putted half a kilo in 500ml solution and stired for a while. It still remains undissolved salt at the bottom, never mind. Filter this in coffee filter and use it for fixing films. But you have to be prepared to wait quite a long time, some 24 hours and the film is fixed at room temperature.
If you wait some more hours I will show you photos of the first film fixed with KITCHEN SALT!
Actualization at 20:40, 19-3-2012:
Kitchen salt passed this test. The film came out with white areas completely transparent after 24 hours fixing time with a very concentrated solution but without solid parts. If nothing else is available you can use salt, be prepared to wait 24 hours at least. After the first hour, you may open the tank at light and control visually the process. That was what I have done without problems, so far.