Roland GI-20, which is a straight interface, or an Axon AX 100 Mk II, which comprises an interface and synth and packs some amazing and unique capabilities besides.

You also need a so-called MIDI or divided pickup such as the Roland GK-3 (there are others) or a synth-ready guitar.

Google "MIDI guitar" and "pitch to MIDI" for other results, but be careful. There is a lot of dated information out there, and some interesting new stuff, too.

R.