You can try REO.com but I have not had much luck with them as most of the listings are old and all the good deals are taken. Individual banks (Bank of America, Chase, Wells Fargo, etc.) also have a section on their website for their internal offerings. They are not easy to find but you can usually do a search for "shortsale" and eventually find them.
Short sales in our area have mostly dried up and there are not many available because the banks see the housing prices are going up and are not approving many of them. I recommend looking through MLS or redfin.com to find anything tagged as a short sale and putting together a list of agents/brokers that deal with them.
Contact these agents and let them know that you are interested in short sales and maybe make an appointment with them to to discuss exactly what you are looking for. Once you have talked with a half-dozen or so they may be able to bring you deals before or as soon as properties are put on the market.