Two gear sources I have used in the past are

  Martin Sprocket and Gear
  3100 Sprocket Dr., Arlington Texas 76015
  tel.: 817-258-3000

  Charles Bond Company,
  11 Green St. Christina Pa. 17509
  tel.: 800-949-0059.

Both companies are have been around for many years, and in fact the
Bond company is over 100 years old.  The Bond company is also a
distributor of Martin Gears, as well.

The main thing that they need to know is the diameter of the old gear
and the number of teeth, so as to establish the diametral pitch and the
pitch diameter.  They have gears available from a number of materials
including iron, bronze, steel and phenolic.  They may refer you to a
local distributor of their products, but I will be surprised if they
don't have a stock gear which will be very close to your requirements.

You may have to bore the center hole, or drill the gear for a set screw
or something like that, but that can be done with a drill press or a
portable drill and vice if you are careful.  Hope this helps.

Jim Cook