Check at vacationstogo.com. You can put in when you'd like to go, where and for how long and it should bring up some choices. They do list some that start and end in Venice and Barcelona. Watch the ratings, too. If it's below 4.5-5 stars we usually don't bother. You can find a lot of reviews at cruisecritic.com and while some people will whine about everything, you can find some pretty good info from people who have taken the cruises you may be looking at.
Of the 3 cruises we've been on from here, 2 left from Civitavecchia, near Rome, but they were longer than 7 days. If you do leave from there, you can either take the train up to Rome, then have to get on another train out to Civitavecchia (kind of a headache), or you can go here to the exchange and rent a car, drive up, drop off the family and the luggage at the ship, then drive about 6 blocks or so and drop off the car. You can see the ship from the rental place and it's about a 10-15 minute walk back to the ship.