Start with a good broker/agent with experience in multifamily. Many (not all) wholesalers focus on SFR, so it can be tougher to find one that does a lot of multifamily volume.
Find a real estate investors group and start networking. Take your time and find investors that are honest, not the guys that tell you they'll cut you a sweet deal the first time they meet you.
Don't have an investor group? Find some local lenders and take them to lunch. See if they can tell you who some of the active investors are or real estate agents that work with investors.
You should be able to find a local investor or two that's willing to share information with you.
I've found that larger (80-units or more) apartment deals that come from wholesalers are usually a waste of time - mainly because the financials (like T12s, Rent Rolls, etc) are incomplete. Because of extensive financials you need to properly underwrite a deal, I have found that developing relationships with the top brokers in your market(s) is a better investment of time/resources than talking with wholesalers.. Multifamily brokers certainly earn their commission and it is worth working with them.