you can google wav, mp3, or guitar to midi, but most of the free stuff is mono phonic.
Or you can get a midi guitar or midi pick-up, also expensive.
Are trying to transcribe original material to MIDI? If so and you go the old fashion route and manual enter the score/tab or midi, I think you will find it will increase your transcribing skills.
For an affordable Transcribing software I have heard good thing about this, haven't personally tried it yet.
Also, I have used Sibelius which includes AudioScore Lite, which can convert an instrument signal into musical score (which easily converts to midi). The full version is expensive, but I have had some success with the lite version, you do have to clean up the note values (for timing) a bit, but it should get you to 80 to 90 % there.