The best (and least time-consuming) sources for us have been cold-calling & email campaigns.

Anyone know any sources of information I could get on this subject?

Also, look into Green Bonds or Green loans if you are adding EV charging or solar etc. as Banks like to access that source of money if you're modernizing a building to be Energy efficient or leed compliant.

Propstream is a great source to find off market multi's in your area and you can mail or skip trace the owners to see if they want to sell.

Next, your realtor or PM should be able and willing to provide projections on the property, using a reliable software or other source.

Does anyone have a favorite company they like to use for post cards that is cheap?

Data sources are from Zillow.com/data, US Census Population growth, and ZIP code data sources (example within tabs). 

I'd start with a mortgage broker that can get loans from a number of bank sources.  

All of this info can be sourced from online platforms and articles, real estate investment meet-ups, and general networking with folks in the industry.Maybe studying all of this is unnecessary, but at the end of the day, the more you can perceive change in the RE space, the better you can adjust your strategy and criteria to profit in the market.

My 4th property I sourced and manage on my own, much more time consuming but much more cashflow too.