Oil Sending Unit

Recently my oil pressure gauge began fluttering from normal to nothing back and forth. It seems to get better when the engine warms up, but still does it a few times per mile. Several professionals have told me that it seems more likely to be the sending unit than the pump because it indicates such sudden changes. My question is, assuming that the problem is in fact the oil sending unit, where is it located? And what is the easiest way to get to it? I have a Haynes manual but I am unable to find ANYTHING about an "oil sending unit"

Thanks