I was after the same a while ago. The only place I could get one was off eBay (OK, may be I didn't look too hard elsewhere!).
I think one of these would do click here or click here - but confirm this with the seller before bidding/purchasing.
What make is your graphics card & was it retail? I'd have thought retail ones come with the necessary cables (my g'card, a ATI 9700, was a secondhand one off eBay by the way). G