How to find off market apartment complexes? Skip Tracing?

[Dave Klein]

I'm looking for a medium sized apartment buildings/complexes on the east side of Wisconsin, specifically northern Milwaukee, Green Bay, Appleton, Sheboygan, Oshkosh or Manitowoc.
I pulled my list and narrowed it down to the 8-32 unit buildings that I'm after. I sent out a mailing to about 1200 back in June and received only a handful of calls all for people looking to get rid of duplexes.
I think I would have better luck calling owners directly. Does anyone have suggestions on a good skip tracing company? I have a great list but no phone numbers.

My theory is that someone who owns a property that large is probably a somewhat savvy investor and not going to pay much attention to post cards.

I'm open to feedback here from those that have successfully found medium sized off market multi-families.