You should be able to find expanding "tote" bags regular tote bags, backpacks, carry on size luggage, larger luggage, etc. on board. They had plenty last October. But definitely be prepared to pay more than $20!
I ended up getting one of the tote bags with the cruiseline logo on it. I put all of my shutters pictures in it, along with the pictures that I had developed on board, and my navigators, etc. It was quite heavy by the time I was finished packing!
Sue Ellen