Having a good agent/broker is always a high priority and great tool for finding and negotiating deals. Having strict criteria can help you and your broker find deals for you much easier. On market deals today will mostly not be very good, so off market may be the way to go.
One way to find off market deals are through a direct mail campaign. Try your best to find motivated sellers, and follow up with them unless strictly told otherwise. Owners who have evicted multiple tenants may be motivated to sell their property.
Another way was basically what I already said with the broker finding you deals. Other members of your team can also be a great source such as your property manager and mentor. This is why it is very important to build solid relationships, so they will actually give you leads.
I have also heard of trying to purchase another property in a surrounding area for every on market deal you come across. You would be purchasing two properties instead of one, and the off market property will give you more cushion to offer more on the on market one.