What parts need to be bent? If it's just the gunwales and keel stringer, there's no need to soak or steam the wood.

Most lumber yards carry spruce in the form of 2x stock, often 2x8 - 2x12. If you want sitka spruce, be prepared to spend a fortume. You may be able to find it at Goose Bay Lumber in Chicester, NH (east of Concord on Rt.4). Jackson Lumber in Lawrence has a good selection, but I'm not sure what they have for spruce. Yankee Pine in Rowley (on Rt. 1, across from New England Small Craft) definitely has spruce, as a friend of mine has built an SOF using it.