http://www.ego4u.com/en/cram-up/grammar/passive
THIS IS A GOOD ONE WITH PLENTY OF EXERCISES. TAKE ADVANTAGE OF IT!
http://www.nonstopenglish.com/exercise.asp?exid=921
http://www.nonstopenglish.com/exercise.asp?exid=657
http://www.learnenglishfeelgood.com/esl-passive-voice-exercise2.html