These are the numbers from the Public Policy Polling survey of Iowa -- done between September 18th and 20th of a random sample of 488 likely GOP caucus attendees, with a margin of error of 4.4 points.
It shows Trump is still leading with 24%, followed by Carson at 17% and Fiorina at 13%. All other candidates are in single digits (8% or under).
NOTE -- The survey was done before Walker withdrew, so I included his 5% in the unsure category.