That's a great question and I think it depends upon the size of the property you're looking for. The commercial real estate companies typically are your best bet for large multi-family properties like apartment complexes however they sometimes list the smaller 2-4 unit multi-family dwellings too. The majority of the 2-4 unit multi-family dwellings are typically found in residential areas, so there are some Realtors out there like myself that do list and sell these types of properties.
So if you're looking for something large, contact a commercial broker. If you're looking for something in the 2-4 unit range, contact a "residential" broker and ask to talk to the principal broker. They'll know the best person in the office to help you since they know the strengths and weaknesses of the agents in their office. Depending upon your experience level, you'll want to look for someone that understands how to determine the ROI (Return on Investment) and will know all the things to look out for when purchasing a multi-family property. Feel free to contact my team if you have any other questions.