First Class Deck plans only in: White Star Line Royal and United States Mail Steamers: First Class Rates and Plans. You should be able to obtain this from THS or 7 C'S Press. I think they may now be linked. The booklet will also give you the Olympic and Majestic. It dates from May 1913.
The General Arrangement plans for Harland and Wolff designed vessels are held at the Ulster Folk and Transport Museum. H&W used to sell copies but since the museum took over responsibility of the plans, I don't know if they still offer the same service.