before starting up my d15z1 swap i replaced the header on it and found that one of the exhaust ports was completely coated in oil
i checked the top of the spark plug to see if it was covered with oil but it was dry
how else would it be getting in there?

 

Most likely a bad valve seal and/or valve guide.

 

how would i check which one it is and how would i fix that?

 

You might be able to pull the valve cover and inspect the valves and find whats wrong. Unfortunately you're usually stuck pulling the head and taking it to a machine shop to have it checked.

 

is this a project that would be easier to have a shop do? and how much would a fair price be for this type of thing?

 

I'm with Kzelisko.

 

Don't assume there is ANYTHING wrong yet unless you actually run it.
The oil is probably a leak by if stored on it's side etc, especially as you say the plug looks ok.