Guest,Eclipse
We have looked at some vague reports that you can get "midi" guitars and such, but they turned out to be more "vague" than "report." Lots of people have "heard of one" but no one we have ever found has actually seen one.
For a clean MIDI input to other MIDI devices or to your computer, whatever kind of "pickup" you use has to determine what note (or notes) are being played. Overtones, and slight variations in pitch make this conversion rather complex if you are trying to "filter down" from any audio source to identify one note at a time, and can reach mind-boggling complexity if you start playing chords. A system that actually detected which fret you were on might work, but it might also make your nose glow when you play the instrument.

Mary In Kentucky
I use a keyboard for most input, but in "single note" entry mode. Select the "time value" and play the note, one note at a time. My program puts the right dot into the score. I find it easier and faster than trying to click the right spot on-screen. If you're as shaky as I am, you may need to reset "on velocities" for the whole score when you get done, but that's a fairly simple step with most programs.
I should note that I use MIDI input mainly to make written scores. The MIDI files that I get are a byproduct that I use mainly to see that I got it more-or-less the way I intended to.

John