It appears as if they buy their spinach from Natural Selection.
From the L.A. Times:
The company (River Ranch Fresh Foods) buys in bulk from Natural Selection Foods, one of the nation’s largest producers of organic produce. Since last week, Natural Selection has been the primary focus of the investigation into the source of the outbreak, because many of those sickened apparently ate its packaged spinach. However E. coli has not been found in the company’s bags of spinach, and health officials have said others could be implicated. Natural Selection issued its own recall of 31 brand names.