Find the power of the real current source?
Just remove the current source. Do not remove R4 (If you remove R4 it will distort the result).

Then determine the Thevenin Equvalent of the remaining circuit: get equivalent circuit of voltage source \$V_{Th}\$ with a series resistance \$R_{Th}\$.

The voltage across the current source then is \$V_{CS} = V_{Th}\$ &plusmn; 1mA * \$R_{th}\$

(The ambiguity of the &plusmn; sign comes from the fact that it is not clear whichg side (A or B) is the GND side of the Thevenin Eq.)

Now you have the voltage across \$V_{CS}\$ and the current through \$I_{CS}\$ (=1mA) the current source to calculate the power.