what you are describing is simple compared to what some of the readers are describing. You just need to buy a reverse pitch prop, also called a "pusher" prop. these are made in various sizes. some of the previous comments thought you wanted two props running off of the same engine on the same axis. I'd search the popular web sites for prop manufacturers:
heres one: www.apcprop.com

they show a pusher prop in 14 and 20 inch diameters. you can cut down a 20 inch prop to smaller diameter if you have to.