Where does the energy come from in a nuclear fission reaction?

First, the exact masses of the nuclei are not the same as the mass numbers in the equations. There are very slight variations in mass, but these variations are exceedingly important. In order to calculate the actual loss of mass, you would need the masses of the various nuclei and neutrons to several decimal places. Then, when you add up all the masses, you see that there is a very small change in the mass during the reaction. Since c^2 is such a huge number, even very small mass changes result in the release of a lot of energy.