Off the top of my head I can't immediately think of a lament from Lewis, but one tune which must be fairly easy to track down would be Macrimmon's Lament, which comes from Skye, not far from Lewis and probably visible from there on a clear day. Alternatively you could try Fear a' Bha\ta (The Boatman) or An Ataireachd Ard (literally The High Surge, but usually known in English as The Eternal Surge of the Sea.) These are not laments but would not sound out of place and should be some of the easiest airs from Lewis to find in print or recorded. I read somewhere that Fear a' Bha\ta was written by a girl from Stornoway, where the Iolaire was lost.
    (I use the back slash to represent a "grave" accent over the previous letter; my computer literacy doesn't extend to writing it properly!)