P12 starter/solenoid problem

Hi everybody, when i try to start my car the starter motor makes a loud screeching noise, the starter motor turns the engine but not always enough to start the car, some times i have to bump start it because when i turn the key often the sreeching noise comes in and the starter motor stops, i think this might be the stater solenoid, if so does anybody know where i can purchase a starter/solenoid repair kit or has anybody else had this problem and how did you fix it, many thanks