Another option would be to try a Swiis made Schertler contact pickup ...
SChERtLER - Audio Transducer
I have the DYN-G for guitar, which fixes onto the instrument with putty...and if well placed, gives a very natural guitar sound. Most Schertler pickups retail for around $400. They can be easily plugged into any xlr mic input. Generally I have found feedback not to be a big problem in using these with guitar.
I know many of the top concert professional concert classical guitarists, like the LA Guitar Quartet and The Assad Duo use the Neumann KM 184 mic, which sell for about $850 each.
John Hedger