Not sure which ship you want but here's a couple of sites that might help you:
http://cruiseadvice.org/CruiseShips/BestCabins.aspx?ShipName=disney+fantasy
https://www.cruisedeckplans.com/DP/deckplans/Disney-Fantasy
Yes, you can do research on the
DCL site, then go to the Costco site to make your booking. Or call them. One thing to know is that the Costco site doesn't always show all the same availability of rooms that the DCL site does, but if you want a particular room that the DCL site shows, call Costco to make that booking. They will call DCL and get it for you.
Also, sometimes the Costco site looks like the prices are higher, but once you put in your membership number and start the booking process, the prices become the same as DCL. It's a system glitch, I guess.
Another thing you can do is book the room yourself through the DCL website or by calling. Then phone Costco Travel within 30 days and have the reservation transferred to them. They'll send you some paperwork to fill out and they get it done in about 3 days. That way, if you prefer to talk to the DCL agent yourself, you can.
Then again, they might not have all the answers, either. We were assured that 9112 on the Fantasy would sleep 5 and it only slept 4. The DIS website had it right, but 3 times I was assured by DCL that it would sleep 5. We didn't need to sleep 5, but we had wanted the Murphy bed.
That leads me to another link:
Once you have narrowed down the deck and some of the rooms that interest you, check out the following thread for some detail:
https://www.disboards.com/threads/d...pdated-list-of-rooms-needed-1st-post.3194826/