My best cell builders are from two sources.
1) If you are fortunate to catch a large swarm, and I'm talking 4 or 5 pounds, they make great cell builders. After hiving the swarm, and I like to cram them in a 5 frame nuc or a 5 over 5 nuc, wait 24 hours and start feeding. Go in, and remove the queen. the best part of this is that no egg in the hive from the swarm is over 24 hours old. So no early or missed queens to kill of the graft. This technique is good for a smaller amount of queens up to 15, although I have had grafts much higher.
2) My second favorite way, it to remove the queen from a strong hive, but net necessarily a large hive. Wait 24 to 48 hours (48 being better), and go in and remove all started queen cells. This allows you to harvest the royal jelly from the queen cells. You then graft back into the hive with their own produced royal jelly. The acceptance rate is much higher, and for times late in the season when traditional grafting and rate decline, this usually puts the numbers back up to 90% take. The only down fall, is that you better not miss ANY queens cells. I will go back in about 8 days after grafting and look again in case I missed any queen cells started by the bees prior to the graft. Using a queen excluder or just placing the cells in a made up nuc with no queen, ensures the cells will not be killed by a rogue queen.