For bigger apartment complexes, you should work with a commercial REALTOR as most residential REALTORs will not have a clue on how to go about helping a client buy commercial real estate.
If you go to sites like loopnet.com (the most well-known for residential multi-family buildings) you will be able to find properties, but be advised that the information is frequently out-dated. You will find several properties that are showing as available, that are no longer available. But it is a good way to start to have a feel for the market.
When I first started investing in Real Estate (even before becoming a full time agent) I researched multi-family properties here in the Phoenix area. It is fascinating and there is a lot of money to be made in that market if you are careful and buy it right.
One strategy that I considered using was to buy a 4-plex, move into one of the units and rent the other 3. You can live mortgage free this way. But there was no way I could convince my wife to go along with the idea. To this day I still think it is a solid way to invest in real estate.
But I digress... Good luck and feel free to ask more questions as you research this market.
Jose Dias, REALTOR
Realty One Scottsdale
Good luck to you and make sure you work with a commercial agent if you decide to go ahead and purchase. Us commercial agents can help with the pro-forma financial on income producing property.
Good Luck to you.
There are many smaller (under 10 units) multi family on the MLS. Your Realtor can show those to you. However the larger apartment complexes are usually not listed on the MLS. Your RE agent that focuses on commercial can find these properties for you. They are not listed in any one place, however there are several websites that many of the commercial multi family apartments can be found on.
Also checking on the websites for the top ten commercial brokers will bring you more apartment complexes.
Jeff Masich, Realtor
Arizona Homes and Land