Water blended with the motor oil

Any auto spare parts out lets or you local dealer. If you do the job your self the cylinder head must be checked for cracks and warp, this will be done by an automotive machinist. You can check on line for local suppliers in your town.

One big question, how long has it been running with coolant in the oil? The bearings in these engines do not like coolant contamination. Before you tear it apart I suggest draining the coolant and oil. Then changing the oil, use cheap oil as you will be changing it again, run it just long enough that the oil circulates, then drain it again. Now connect a mechanical oil pressure gauge and put in the new oil, start it and check the pressure. You want to see 40 psi minimum with cold oil and engine.
Less and I would look for a used engine or get a re-manufactured one and use yours as a core. Or if you wanted you could rebuild yours.

The most common area these fail is at the intake manifold gaskets. Courtesy of the redesign of the intake manifold bolt placement causing poor clamping force as well as the change to a composite type gasket.