I would suggest Clear or Promote http://www.detoxologie.com. It will promote regularity while gently cleansing your colon. Clear mixes well in fruit juice which will fit nicely into your fast. Promote is encapsulated for your convenience. They are both very gentle and yet effective.
If you are looking to purchase a fiber product at your local health store, look for a gentle bulking agent that will promote regularity with out stimulants like senna. You don't want diarreah and cramping.
Good luck with your fast. Keep me posted on your results.