A bill of sale is required on title transfers between individuals. A purchase agreement is required if the purchase is from a dealership.
Find Comprehensive list of Iowa Motor Vehicle forms at
http://www.iowatreasurers.org/iscta/access/commonNavigator.do?idContent=118
More forms available at the Iowa DOT:
If you want a numbered (non-personalized) black out plate, please bring your current plates and registration to either office to trade for the new black out plate. The fee is $35.00 initially and $10.00 annually.