You basically need two elements; a hex pickup and a converter.
For pickups you have the choice of the Roland GK-2 or GK-3 ranges, the Yamaha G1-D (all of which are
magnetic pickups that sit next to your bridge), or the RMC or Graphtech Ghost piezo systems. The piezo systems are much more expensive, but offer better tracking and accuracy.
For the converters, Roland do the GI-20 or the GR series synths (which also have MIDI out). There's also the older GI-10 which is fine for most work. AXON do some uber-fancy ones, but they're horribly expensive, and there's the Yamaha G-50 which is supposedly superb if you can find one secondhand.
