We don't have a problem keeping duplexes rented in B-/C+ neighborhoods, but our mgmt company also has 20 years of renters in a database, and do a great job of vetting.  

Search their online database for crimes in the area.

We buy and dozens to hundreds of properties per month and have done so since the late ‘90s.Around 2005 I had a SQL relational database custom designed by some bright local guys to the tune of about $300K to ease the processing pain my staff was experiencing.

They are eager to build their contact databases.

I would go the digital route for your application, there are a ton of benefits there, easy to manage, integrated with screening reports, no retyping tenant info into your database or to pull reports, plus it's usually way easier for tenants to fill out.

If you have the address, do a google search for the town and "assessors database" or "assessors maps"  once you're in there, you can type in the address and it will give you the municipalities info on the property.  

They'll generally have a few off market deals, or will put you in their database and will contact you when one pops up.

This can be filtered through the many different data base websites out there.

Even if there was a database of unit mixes, I would still have to search individually so calling would just be an extra step.

HUD has 2 databases of 203k lenders that can help give you a good start, and I'm happy to help you locate and navigate them.