I think part of your difficulty is that the term is not a medical or anatomical term really, so you won't find a concrete illustration of it. It's a little like "lumbago," a loosely-defined entity from the days before we had our precise tools to identify and make images of structures. SP does have an amorphous correspondence in the human body, as does lumbago for the back ("lumbar") region.

Having said that, here's what Wikipedia [for what it's worth] has to say about the Solar Plexus: http://en.wikipedia.org/wiki/Solar_plexus. Some is acceptable and some phrases are meaningless from a medical standpoint.