how to find thevenin equivalent of this circuit via source transformation?
Just as an aside in order to find the Thevenin equivalent of the right side you need to use source transformation for three times. First find the Thevenin equivalent of the current source and the 40k resistor. This gives you a \$V_{th}\$ and \$R_{th}\$, with \$R_{th}\$ being in series with the upper 20k resistor. The next steps would be to find the circuit equivalent of the left 20k resistor and the circuit right to it, which you've just figured it out. This involves two separate steps to finally reach the Thevenin equivalent of the right side. /end