We found the best time for ice delivery was in the morning between the time it got to hot to sleep and the main stage opening. Just hang around as you normaly would at this time and you will probably see the guys on ATV's selling ice.
What we did last year for ice was take advantage of the easy VIP re-entry. You can easily leave and re-enter the Bonnaroo site all you want via the VIP entrance all weekend long. We just climbed in the car every morning after everyone was up and went out for breakfast, ice, and beer. The ice we picked up at Wal-Mart was much cheaper than inside Bonnaroo. Another nice advantage to this is since we went out for beer each day we did not have to pack all the beer we needed for the entire weekend which saved us a lot of space in the cars.