Chris,
I have done a little bit of gearcutting when I made new gears for an old Myford lathe. I had to make a new bull gear and another one for the back gearing setup.
For Horology (clocks) they usually manufacture their own flycutters and gearcutters to make the gear setups, and that is all usually done on a small lathe with a milling spindle setup.
http://www.csparks.com/watchmaking/WheelCutting.htmlFor involute gears, which is the normal type we use, and the ones I made, you buy your cutters ready made. Usually the sets come with eight cutters, to cover the whole range of teeth required. I just bought the ones that were needed.
http://rdgtools.co.uk/acatalog/INVOLUTE_GEAR_CUTTERS__SET_S_AND_INDIVIDUAL_.htmlThey are fairly easy to do if you have the equipment. The major bit is either a rotary table (and you need to know how to use it correctly to obtain minutes and seconds of a degree), or like I did, a dividing head, which makes it a lot easier. A very rigid arbor to hold the cutter, and a set of tables (usually in engineer tables) to give you the correct diameter of blank and depth of cut for each tooth. Once you have it all set up, it is just a normal milling procedure. But usually a rather tedious one with no disturbance required. Make a mistake, and it is start all over again.
If you want a lot of the same size, you could cut the gear profile along the length of a bar, and part off as required.
For one off's it is better that you use a gear stockist and buy individual ones, as to buy all the gear cutter sets to do every type and size of tooth form would cost a fortune.
Unfortunately, the ones I wanted were not produced, so I had no choice but to make my own.
John