Eric, what kind of mic are you using at the moment? Also as you are probably aware you can spend as much as you want on microphones. Giving me some idea of your upper limit would help me make a recommendation.
I concur with Adam – a bit more detail would help in what is a very large range of possible answers. Mics have various uses with quite specific outcomes. Can you give some detail on how your harp sounds? (type of harp and wood made of if a folk harp). What genre of music?. Concert hall sound or in close and familiar?