overheating problems

I have a 87 caprice it just started overheating yesterday, also when I took it for an oil change they mentioned that my oil was kinda mixing with water, even though another mechanic said it was not and all I needed to do is change was my lifters (tappets).
that has been a while and then all over sudden the car starts to heat up, I wanna change the thermostat for a start then go from there.
My only dilema is where do I find the thermostat! is it easily accesible. with my ride overheating I am losing too much power and my oil pressure light is coming on but I do have enough oil!! If anyone has any ideas please help Thanks in advance

if the oil press. is dropping it is prob what is causin the over heat if you cant do it yourself take it to a mechanic and have him give it a look if your oil pump is goin out it will destroy your engine so do it soon and dont take any long rides or it may go boom

The thermostat is inside the housing where the top radiator hose connects.

Water in oil could be a leaky intake, blown head gasket, or other stuff.

Check the oil. If it is kinda milky, you have water in it.