I made my own magnetic pickups in the following way. First, I pulled apart some small stepping motors that were discarded at my workplace. Each motor contains two little coils on nice plastic bobbins. I found some little rare earth magnets that fit inside the coils.
For my housing, I cut apart pieces of 1/4 inch thick Lexan, from the scrap pile at home. Filed to shape, tapped, with little #4-40 machine screws. Quite a contraption, but it's not too conspicuous at a distance.
For better or worse, I don't have any more of the coils or magnets. I know that there are places on the Web that sell little rare earth magnets. Digi-Key sells a $15 stepping motor, and you would need two motors to extract the four coils. Choose "bipolar" motors, and the highest possible coil voltage, which corresponds to the most turns of wire. The only remaining variable is diameter. The Digi-Key motors are of a type that have two metal shells welded together. You have to break the welds by gently going through them with a hack saw. Going a little further with the hack saw starts cutting the coils themselves, if you catch my drift.
Even using pre-made coils, if jflojazz could make this work and sell it for $200 with a decent profit for himself, it would be pretty cool.
Now for a technical observation based on a minor mistake that I made. In the series humbucking mode, you should make sure that you alternate the polarity of the magnets along with the polarity of the coils, so that adjacent strings will be in phase with one another. This is not an issue when using the pickup by itself, but is necessary if you want to mix the magnetic pickup with a piezo.
Oh and one other thing... you can get away with fewer turns of wire if you don't mind building a small low noise preamp to make up for the reduced gain. It could be a simple JFET affair, and doesn't have to be mounted close to the pickup.
