ebay is a great idea if your just looking for a used one. A lot of people switched to Lite, so you might be able to find a used DS for cheap. Some even come with games, which, even if they suck, you can sell
and if you don't feel like dealing with auctions, just try looking for one used. They're in low demand, so it shouldn't be too hard to find a good deal on a used one (same thing happened to the regular advance when SP came out)