Have you not tried using a cruise comparisom travel agent that gives you the opportunity to enter the month you want to go and the port you want to sail from - and see what results come back? Try Voyan.
Have you looked at Royal Caribbean, they do 2 itineries on Splendour of the seas,departing from Venice which you can purchase as cruise only. We did one of the itineries at the end of May this year and purchased our own flights to and from Venice. We visited Dubrovnik, Turkey, Santorini and Corfu, many of the other passengers on board were doing both itineries as a back to back.Good luck.
I think that for a request like this you ought to be talking to Cruise Agents. There are certainly cruises that you want and you will be able to get them "Cruise only" but you have to talk to agents or even direct with the Cruise Lines. People who respond to questions here are just experienced (and even inexperienced ) passengers and/or anoraks....Neil Dow.
The specialists for that area are Costa cruises -see their website and you certainly can easily book cruise only. They cruise mainly from two ports Savona nr Genoa or Venice - which is a magic departure port due to the fabulous cruise past St Marks Square and through the Venice lagoon. Costa is a great cruise line with super modern, clean ships but only go with them if you appreciate a mixed culture ship and mixing with other nationalities but if you only want Brits or Americans make another choice.