Resource for locating multi family properties/apartment buildings

[Bruce Clark]

I currently own several single family homes and small multifamilies (duplexes and triplexes). I am preparing to purchase a larger multifamily property. At this point I am thinking I would like between 10 and 20 unit property. However, I am just starting my education and research phase. 

What websites and online resources are available to locate properties in this category. Obviously realtor.com is of no help with regard to these assets. I regularly follow loopnet, but it clearly does not provide a great selection. Are there other online resources that I should be using?