From what I have read, Yes they can be rented prior to sailing. It is on the web site at carnival. I think customer (something) then you can click on rentals from there. A friend of ours that went on your last cruise rented one as soon as he got on the ship. So that is an option also. My husband rented one from our local shop and will be able to bring it with us, for less than Carnival wanted. You might want to check that out also. We were just concerned with the what if's? wrong size? wrong color? Didn't have it?
That is what we did.