Some tools myself and others have used: - Skip tracing- Mailers- Data aggregators like PropStream, PropertyShark, etc. 

Out of the 20 letters I've sent, I've had 1 guy get back to me and we were in discussions but weren't able to find a price that worked for the both of us.The issue is that I only have a few houses on my mailing list so far (just places I've walked or driven by that caught my attention), and blindly searching every street and address # to see which houses are actually legal multifamily will take forever and doesn't seem efficient.I know the data is publicly available, I just don't know how to aggregate it in a way that would be efficient and useful for blasting out marketing mailers.Are there any websites that can pull massive lists of all multifamily houses in my area?

After that, I would encourage you to go to your county recorders office / court to request data yourself.

Also advertising costs.On loans, I would hunt for an 5 year fixed interest only with low pre-pay penalties that you can refi when rates finally come down (hopefully).Landscaping / snow removal is legit and is more viariable than you would expect.

Finally wealth creation, this I'd do it for.

They would never look at 99% of the 154 units that are listed on Zillow at any given time.I used this exercise to show how opportunities fall through the cracks & most investors make investment decisions based on bad or irrelevant data.

Then cabinetry, countertops, and final coats of paint (I have the base and case pre painted before install so that caulk and paint touch ups for the nail holes are all that is needed).

But with Intel's $20 billion plant, Google's three data centers, and land buys by Amazon and Meta, Columbus is worth it!

Propstream is great, you can get a lot of data off of there as well as driving for dollars which I believe is also around $99/month. 

Do you rely on specific data sources, gut instincts, or perhaps a blend of both?