EVERY Real Estate Company's: Listings, Foreclosures, Short sales, Estate sales, Bankruptcy, Pre-foreclosures... etc...
Duplexes are (in my opinion) much better investments than Fourplexes. Fourplexes are much more management intensive... and have higher vacancy rates than Duplexes... and are usually not in the "best" neighborhoods. The only problem I see with Duplexes is that most of them have not been keep up too well. Most investors are not experienced with the realities of rental properties, as can be seen in the relatively short time a Duplex (or Fourplex) is owned! Generally I believe Duplexes are over priced by the inexperienced ("Wanna-Be Investors) paying too much. Be careful! Make accurate cash-flow projections before buying!