A lot of duplexes, triplexes, quads and small apartment complexes are listed on Triangle MLS. Bigger apartment complexes are generally handled by commercial brokers and appear in data bases like loopnet and TCAR. If you are interested in working with a broker, I would be happy to discuss with you in greater detail and develop a strategy to help you find an appropriate property.
Clarence I or any agent you wish to work with can send you the duplexes and multi-family homes that are listed. I can also refer you to a commercial agent if you are looking to purchase large rental apartments buildings, however most of those generally do get listed as there are often people already known that are waiting to make those investor purchases.