i think i may have sad

hi everyone,
i realise that this topic has been raised many times on this forum before, but it may be relevant for others too. i've been reading posts on the forum for a few months now, finding myself to have similar symptoms to many others.
i never really considered it until my girlfriend suggested that i may have sad, as i get hit with a slump/depression on a fairly predictable basis. it usually starts late august, or even early august and lasts until spring.
in a way it has helped me understand why i have acted the way i have in the last few years, though i have not been to a doctor for a diagnosis, sad seems to be the problem.
the point of this post is to ask how others prepare for and cope with winter. as a 'new' sad sufferer i need all the help (however basic) i can get. if anyone could advise me on a good value lightbox (i'm a student) and any sort of routines, diet and other things that may help i would be really grateful.
i'm on holiday at the moment and am anticipating a slump on my return to britain. so again, any help that anyone can give would be greatly appreciated come winter time. thanks for reading this, and thanks for your time.
sandalau