Just a note. The MLS does not carry all multi family and commercial property. You need an agent who is also a member of CBA, the Commercial site. Not all agents are members of CBA and many commercial and mulity family properties are only listed on CBA by agents who never do residential real estate.
I do not believe there is a public site that makes these properties available on the internet. Only those that are listed in with the residential property will show in the public sites.
1-4 units are considered the same as residential for financing and most of these are in the mls, but sometimes there area few in CBA that cannot be found elsewhere. 5 units or more are not covered by RESPA and so if you are a first time investor purchase a building with 5 or more units, you will want to research the differences once you get over 5 units.
I do not generally do investors, except people I know. But just wanted to give you that heads up so you didn't miss seeing the bulk of the properties by looking in only the mls.