I was just reading about this last night in the book Take Charge of Bipolar Disorder by Julie Fast and John Preston.

What you want is Omega-3 fatty acids, for which one source is fish oil, e.g. cod liver oil. Other supplements that may be valuable are B complex vitamins and vitamin D (sunshine substitute). Everything I've seen says to make sure you check with your doc before starting with anything, though. So I'll be asking mine about these supplements next time I go in for a med check.

Light boxes are not cheap. Google or Amazon will show you some options and the price range. The cheapest one I saw on a quick scan is $80 and they ranged up to $400, probably more depending on the type. The bulbs would be expensive too, but if it helps, it's probably worth it. The light box itself is a one-time investment, and sometimes insurance may cover it if it's prescribed. However, the cautionary note about light boxes that came from the aforementioned book is that too much light can trigger mania. Mania hospitalizations peak in summer, and depression hospitalizations peak in winter. So be careful!

I'd recommend consulting with your pdoc on any of these complementary therapies, just to make sure you've got your bases covered.