Many of the short sales on the market now have a pending foreclosure sale date and an experienced agent can negotiate with the seller's investor to request a postponement of the sale. That's not guaranteed but often approved especially if it's a good offer and appears in the investor's best interest to wait for the short sale to occur rather than take it through to foreclosure.
If the home is NOT on the market, there are ways to approach a homeowner to ask if they'd be willing to sell their home. There's more sensitivity required here. The homeowner is obviously in some difficulty and not excited about losing their home even if it's inevitable.