What is the source of the electrons generated in the Krebs cycle?
The two electrons and a proton of NADH comes from a H$^-$ ion of a Krebs cycle intermediate during Isocitrate dehydrogenase reaction and Malate dehydrogenase reaction. 

In the following reaction, Isocitrate is oxidised to Oxalosuccinate and in the process two Hydrogens are lost one as H$^-$ and the other as H+. This H$^-$ is accepted by NAD+ to form NADH.

Note: Oxalosuccinate is an intermediate formed during oxidative decarboxylation of Isocitrate.



Now coming to FADH$_2$ $-$

During oxidation of Succinate to Fumarate, Succinate looses a H$^-$ and a H+ ion which constitute the two protons and electrons of FADH$_2$  . Here's an image that illustrate the mechanism:



The alpha-ketoglutarate dehydrogenase complex reaction is similar to pyruvate dehydrogenase complex reaction(PDC). 
PDC reaction is an oxidative decarboxylation reaction of Pyruvate in which the 2e$^-$ of NADH are derived from the electron shared between COO$^2-$ and C=O groups of Pyruvate. 


The Hygrogen (H+) of NADH is either derived from the H+ of CoA-SH or from the medium. There's a series of complex reactions which need to be studied to fully understand the source of H of this NADH.

So you can understand that the 2 e$^-$ of NADH produced in Alpha-ketoglutarate dehydrogenase complex reaction are the electrons between COO$^2-$ and C=O groups of Alpha-ketoglutarate and the H is either from the medium or the H+ of CoA-SH.

Reference: Biochemistry by Berg