SCI encourages you to purchase materials from your local yarn store. A network of these stores across the United States designate themselves as SCI Knit Spots. They have Christmas at Sea patterns, stock the right yarn (or know where to get it), and can answer questions about knitting for Christmas at Sea.
If you do not have access to a local yarn store or prefer to order supplies online, Jimmy Beans Wool offers yarn for Christmas at Sea projects at a discount. Find the yarns appropriate for Christmas at Sea projects here. First-time buyers receive a free shipping coupon for their next order and a 5% credit in their account; for subsequent purchases, knitters and crocheters receive 20% off after submitting a completed CAS project—just enclose the Jimmy Beans Wool receipt with your submission. (N.B. The 20% discount applies only to Christmas at Sea-appropriate yarn.)