Why gate - source and not gate - body voltage?
The electrons come from the source, which is an N-doped region. This source quotes Chenming Hu's book "Modern Semiconductor Devices for Integrated Circuits":


  ...there are few electrons in the P-type body, and it can take minutes for thermal generation to generate the necessary electrons to form the inversion layer... The inversion electrons are supplied by the N+ junctions...


I also ran across a (probably illegal) PDF copy of Ali Niknejad's book "Electromagnetics for High-Speed Analog and Digital Communication Circuits". I'm not going to link to it, but here's the relevant quotation from section 2.3 regarding MOS capacitors:


  In the above discussion you may have wondered, “Where do the electrons come from
  to form the inversion layer?” In the body of the MOS-C structure, electrons are minority
  carriers and few and far between. So when inversion occurs, where do we find all the
  electrons necessary to invert the surface? Well, there was a subtle assumption that if we
  apply a change in gate voltage, we wait long enough for thermal generation to create a
  sufficient number of electrons to form the surface layer. We may have to wait a very long
  time! In other words, if we apply a fast enough signal to the gate, there isn’t enough time
  for the minority carriers to be generated and thus the capacitance remains at the low value
  given by depletion.
  
  While the depletion region can respond very quickly to our gate voltage since it is formed
  by majority carriers, the minority carrier generation is slow. There is a simple way to solve this problem, as shown in Fig. 2.25, where a n+ grounded contact is placed adjacent to the gate. Normally electrons are prevented from entering the body, like any good pn-junction. But as we raise the surface potential, electrons can easily diffuse into the surface of the structure. Since the energy distribution of electrons in thermal equilibrium is exponential, changing the potential barrier linearly results in an exponential increase in the number of electrons that can cross the n+-surface junction and likewise an exponential increase in surface conductors.


I redrew Figure 2.25 to illustrate what he's talking about. You can see how similar this is to a MOSFET:



In an NMOSFET, when \$V_S &gt; V_B\$, you get a secondary effect called the body effect or the substrate bias effect. This effect acts to increase the threshold voltage. According to Wikipedia, the body is acting more like a second gate.