Is there any good websites or apps? 

Should the property garner no interest from their buyer list, their Realtor list, their social media posts, their offering on BP, LinkedIn, FB, various property investor websites, yard signs, etc. then they would probably be willing to share their fee.  

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?

looking at their website I think investors would get a level of confidence..

I would check out his website. 

website is www.federa.com   it looks like they sell foreclosed and pre-foreclosed properties? 

In my area there is a great landlord/tenant handbook on the county website that is a great resource, perhaps your area has something similar and I'd get super familiar with that.

2.Downloaded a list of multi family owners from Listsource.com3.After receiving the list I searched for phone numbers and began looking up every property (assessors website and Zillow) and becoming familiar with the neighborhoods.

For your property websites, weather if its a direct booking site or not, what website builders are you using?

Vito - Have you checked to see if there is an I-card for the property on the HPD website?