I just don't want to buy any more fonts at this time
Damn, there goes my suggestion: commission a custom serif version of Source. :->
But here's a good lead from Paul's Typblography article*: "I was drawn to the forms of the American Type Founders’ gothics designed by Morris Fuller Benton." Like see if somebody has made a serif version of News Gothic. BTW that sort of thing is FontBureau's neck of the woods.
* http://blogs.adobe.com/typblography/2012/08/source-sans-pro.html
hhp