Allygirl- They provide the shoes with the Tux rental.
WFinn- I did this on the Mariner, it was very easy to do and reasonably priced. The Tux will be waiting for you in your cabin. If you do a quick fitting when you arrive, it you need any changes in sizes you can get them to change before the ship leaves port or from their onboard stock.
The on-board rental includes two shirts, so you can wear the outfit on both formal nights of a seven day cruise. You just leave everything out the last night and they pick it back up.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007