We actually just bought a house doing this exact thing. There was one particular area that only had about 30 houses in it and they don't come up for sale that much. Since we really wanted to move this year, we sent letters to the houses we'd want in that area. To our surprise, out of the dozen or so letters we sent, four people responded that they were considering selling and a few others still replied thanking us for the letter but weren't going to sell.
As long as you are willing to be flexible with move date, this works out great for both sides. The sellers avoid agent fees, which at 5 or 6% can be significant, and the annoyances of showings and open houses. It also gives the sellers certainty of sale so when they go to buy another house, they don't have to deal with contingency offers.