If you're going to "upgrade", I would go all the way to a new Shimano 105 or Ultegra hub and 8 or 9-speed cassette. If you want to stay freewheel, Ebay is one way:
http://www.ebay.com/itm/Dura-Ace-Hub...-/151063354756

It's hard to tell what you intend with this. If you're modifying the bike for practical use, I would stay away from old Dura-Ace cassette hubs. The cogs for them are becoming hard to find. I would only go that direction for a period-correct restoration.