Yep, natural light lamps are lovely to have, but the intensity of a regular light is completely different from a lightbox (which is basically drug-level light; a few people actually get agitated or manic if they use the light for too long).

D3 sounds like a very smart idea. The other thing that might be worth trying is an Omega-3 supplement (fish oil if you're not vegetarian, or one of the vegetarian alternatives) -- there's a fair amount of evidence that they can help mood.