Try Kevin's encoder code, found here:
Kevin's Code
Take the encoder code, and take a look at that. A few changed need to be made (don't know what specifically) and the encoder.c and encoder.h files need to be added to the frc code for it to work.
To order encoders, try digi-key. Aim for a good pulse per revolution, and make sure you don't try to get too much feedback from encoders (try to stay at around 1000? maybe 1500?) and goodluck!