Since Futurama is done differently than traditional animation there are no cels per se. An animation cel is created when somebody takes the image that was drawn by the animator and traces it onto a clear acetate medium. Somebody then comes over this blackline drawing and adds the appropriate colors. These cels are then matted to a background and photographed thereby creating a single frame of animation.
Futurama starts off pretty much the same. An animator draws the image but it is then scanned into a computer where it is touched up, painted and composited with the appropriate background. Since it is done on the computer there are no cels that are produced.
To get around this problem somewhat there is or was a program that the animation studio house were running whereby you could purchase a production drawing. That production drawing had a serial number. You could send it to 20th Century Fox and they would create what is called a one of one. They would create the cel image and composite on the appropriate background for that scene creating a psuedo cel print.
20th Century Fox recently stopped doing this but you can still get production drawings. I have been told by animation houses that if you have multiple images you want one of one cels made of they will consider doing them for you. Probably in the neighberhood of 4 or 5 drawings.
Hope that helps.